ParameterGrid enhancement

[tomaskontrimas]

The ParameterGrid class could be merged with the IrregularParameterGrid, as there is no fundamental reason why it has to have a regular grid.

TODO:

• Update Parabola1DGridManifoldInterpolationMethod interpolation to support an irregular grid:

  skyllh/skyllh/core/interpolate.py

  Lines 672 to 673 in 6c3e66f

  	a = 0.5*(M0 - 2.*M1 + M2) / dx**2
  	b = 0.5*(M2 - M0) / dx

• Make sure that it does not return negative PDF values. It could be done by flooring them to 0 and renormalizing the PDF? Zero gradient could create issues

[martwo]

Special care has to be taken when rounding floating point numbers. With a regular grid it is easier to make sure that all values are rounded to the specified decimal precision. I guess this can be achieved also with an irregular grid. But I would recommend to add excessive unit test coverage to ensure correct behavior.

If the rounding of a value is not consistent to fall on exact grid point values, the interpolation / PDF value lookup will fail, most probably for only a few values.