Fitting based on size distribution

[AnitaZhang0526]

Following some discussions, we think it will be a good idea to add a method of fitting based on size distribution. This has been implemented before in IRENA https://doi.org/10.1107/S0021889809002222
, and the main task would be making the methodology compatible with SasView.

In IRENA, the idea is to find the size distribution that creates the best-fitting curve for the scattering intensity data given certain constraints. The distribution is not limited to Gaussian, lognormal, etc. It is as flexible as the data demand. Specifically, we are interested in using IRENA's maximum entropy approach. More details can be found in IRENA's documentation here: https://saxs-igorcodedocs.readthedocs.io/en/stable/Irena/SizeDistribution.html#model-maxent.