Splitting apart DropSizeDistribution

[josephhardinee]

@nguy I've been tossing around the idea for a while of pulling out a lot of the operations from DropSizeDistribution object and moving them into library functions. This would simplify the DropSizeDistribution object quite a bit, and make testing easier. It would also make it easier to do things like have an operation on a dsd object (like a select) return a new dsd object.
The functionality I had in mind was moving scattering to it's own submodule, and similarly for the rain fits that have been in the object just as a part of it's legacy.

[nguy]

Not sure how I missed this. My bad.

I'll think on this a little, but on the surface I agree. The more modular the code, the easier the testing.