155 mc phase fit

[jrob93]

Fixes #155.

Added a resample flag to PhaseCurve.FitModel. This flag triggers a montecarlo resampling of the observations, using a gaussian distribution centred on the magnitude value and with std equal to the photometric uncertainty. This then returns phase curve parameters with value equal to the mean of the MC distribution, and uncertainties equal to one std of the distribution.

Also includes some small tweaks to ensure PhaseCurve handles units better when they are passed.

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does adler run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?

[jrob93]

I have merged all recent updates into this old PR. The PhaseCurve FitModel function now has a resample argument, when passed it will resample observations within their (assumed gaussian) uncertainties. Currently there is a basic demo at notebooks/mc_phase_fit_testing.ipynb but I will want to add this to a readthedocs notebook at some point.

I also took the opportunity to add more PhaseCurve unit tests