adding dev doc file

docs/dust_extinction/dev_model.rst

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+##################
+Base Model Classes
+##################
+
+All the extinction models are based on either provided in dust_extinction
+or an astropy.modeling `FittableModel`.  
+
+For examples of how all the classes in ``dust_extinction`` are used, see the
+implemented models.
+
+.. _allmods:
+All
+===
+
+All of these classes used in ``dust_extinction`` are based on the 
+`Model <https://docs.astropy.org/en/stable/modeling/>`_ astropy.modeling class.
+See the astropy docs for the full details of all that is possible with this class.
+
+All dust extinction models have the following:
+
+* A member variable `x_range` that that define the valid range of wavelengths.
+These are defined in inverse microns for historical reasons.
+* A member function `evaluate` that computes the extinction at a given `x` and
+any model parameter values.  The `x` values are checked to be within the valid `x_range`.
+The `x` values should have astropy.units.  If they do not, then they are assumed
+to be in inverse microns and a warning is issued stating such.
+
+FittableModel
+=============
+
+The ``dust_extinction`` shape models are based on the astropy `FittableModel`. 
+One general use case for these models is to fit observed dust extinction curves.
+See :ref:`fit_curve`.  These models follow the standard astropy setup for such
+models.  This includes defining the parameters to be fit with the `Parameter`
+function.
+
+Thus all `shape` models have:
+
+* The member variable `x_range` and function `evaluate` (see :ref:`allmods`)
+* Member parameters that are defined with the astropy `Parameter` function.  
+This includes default starting values and any standard bounds on the parameters.
+The number and name of the paramaeters varies by `shape` model.
+* The `evaluate` function that calculates the extinction curve based on the 
+input parameters.
+
+BaseExtModel
+============
+
+The :class:`~dust_extinction.BaseExtModel` provides the base model for all 
+the rest of the `dust_extinction` models.   This model provides the 
+`extinguish` member function (see :ref:`extinguish_example`).
+
+All of the `average` models are based on `BaseExtModel` directly.  Thus 
+all the `average` models have:
+
+* The member variable `x_range` and function `evaluate` (see :ref:`allmods`).
+The `evaluate` function may interpolate the observed average extinction curve or 
+it may be based on a `shape` fit to the observed data.
+* The member function `extinguish`.
+* A member parameter `Rv` that gives the ratio of absolute to selective extinction
+(i.e., R(V) = A(V)/E(B-V)).  This is not set with the astropy `Parameter` function 
+as is included for reference.
+* Member variables that give the tabulated observed extinction curve as a function
+of wavelength.  The variables for this information are `obsdata_x` and `obsdata_axav`.
+The accuracy of this tabulated information is given as `obsdata_tolerance` and this
+is used for the automated testing and in plotting.
+Some models also have an `obsdata_azav_unc` if such is available from the literature.
+
+BaseExtRvModel
+==============
+
+The :class:`~dust_extinction.BaseExtRvModel` provides the base model for all 
+the ``dust_extinction`` models that are depending on `Rv` only.  `Rv` is the
+ratio of absolute to selective extinction (i.e., R(V) = A(V)/E(B-V)).
+
+These are the majority of the `parameter_average` models and they have:
+
+* The member variable `x_range` and function `evaluate` (see :ref:`allmods`).
+The `evaluate` function that calculates the extinction curve based on the 
+`Rv` value.
+* The member function `extinguish`.
+* A member variable `Rv` defined using the astropy `Parameter` function.
+* A member variable `Rv_range` that provides the valid range of `Rv` values.
+* A validator member function called `Rv` tagged with `@Rv.validator` that validates
+the input `Rv` based on the `Rv_range`.
+
+BaseExtRvfAModel
+================
+
+The :class:`~dust_extinction.BaseExtRvfAModel` provides the base model for all 
+the ``dust_extinction`` models that are depending on `Rv` and `fA`.
+
+These `parameter_average` models have:
+
+* The member variable `x_range` and function `evaluate` (see :ref:`allmods`).
+The `evaluate` function that calculates the extinction curve based on the 
+`Rv` and `fA` values.
+* The member function `extinguish`.
+* Member variables `Rv` and `fA` defined using the astropy `Parameter` function.
+* A member variable `Rv_range` that provides the valid range of `Rv` values.
+* A member variable `fA_range` that provides the valid range of `fA` values.
+* A validator member function called `Rv` tagged with `@Rv.validator` that validates
+the input `Rv` based on the `Rv_range`.
+* A validator member function called `fA` tagged with `@fA.validator` that validates
+the input `fA` based on the `fA_range`.
+
+BaseExtGrainModel
+=================
+
+The :class:`~dust_extinction.BaseExtGrainModel` provides the base model for all 
+the ``dust_extinction`` models that are based on dust grain models.  All these 
+models are provided as tabulated data tables.
+
+These `grain_model` models have:
+
+* The member variable `x_range` and function `evaluate` (see :ref:`allmods`).
+The `evaluate` function thats interpolates the model extinction curve.
+* The member function `extinguish`.
+* A member parameter `possnames` that is a dictonary with a key that is a tag for the
+model (e.g., `MWRV31`) and a tuple that is (filename, Rv).  This key is used when 
+initialized a `grain_model`.