Add flexible makemodel to EXP (or pyEXP)

[CarrieFilion]

There are working makemodel and makemodel_empirical Python functions that exist in the pyEXP-examples/How-To/Recipes/Basis example, but I think that there should be a analogous flexible makemodel and makemodel_empirical functions formally included in either EXP or pyEXP. This would make EXP/pyEXP contain everything that one needs to run simulations. This could just be expanding the MakeModel code that exists within EXP or moving makemodel and makemodel_empirical from the examples into the pyEXP code.

Of course, I can always just link to the relevant pyEXP example, but I really do think that it would be nice to have everything self-contained

[michael-petersen]

I agree that some sort of functionality to create input models is important to include with EXP. It would make sense to have it be a function inside pyEXP I think; this looks to require a bit of code gymnastics.

makemodel is simply a flexible wrapper that will take an input density profile function, probably this is all that we need? makemodel_empirical is a bit dangerous and would benefit from some additional checking that we may not want to support.

[The9Cat]

Yes, it would be nice to have that within pyEXP. There are two ways of doing that:

1. Write it in C++ and bind it to the module with pybind11
2. Write it as a function in a primary pyexp module and import the C++ bindings into that

Since there is not currently any direct Python code, I'd prefer (1), basically because I don't want to change everything at this point to support (2). Although that might be the most flexible solution in the end.