Add SIMULATION_FIT_PARAMS to top-level imports from diffstar.diffstarpop

[aphearin]

With this PR, you can now do this:

from diffstar.diffstarpop import SIMULATION_FIT_PARAMS
print(SIMULATION_FIT_PARAMS.keys()))

odict_keys(['smdpl_dr1_nomerging', 'smdpl_dr1', 'tng', 'galacticus_in_situ', 'galacticus_in_plus_ex_situ'])

Previously, you could only import this by drilling down into diffstarpop.kernels, and the dictionary had a very long and non-intuitive name (DiffstarPop_Params_Diffstarpopfits_mgash). I have not changed anything besides defining SIMULATION_FIT_PARAMS to be equal to this obscurely-named dictionary.

[alexalar]

This looks good, feel free to merge!