theory-based DataSource mocks + power spectrum dependency

[nickhand]

The code for the Gaussian, lognormal + Zel'dovich mocks themselves has been done for awhile -- the issue is that they all start from an input linear power spectrum. The user could pass this in to the DataSource, but the cleanest solution is to use our existing cosmology framework and generate it from that.

I think I can actually extract a basic version of my python wrapper of CLASS to do this; it would be a useful community tool either way. We could then include this external package like an optional dependency for certain DataSource classes, similar to halotools.

If I did this, I should probably make the CLASS wrapper interface well with the astropy cosmology class, since it could be more broadly used, i.e., something like an astropy-affiliated package.

Thoughts, @rainwoodman?

[nickhand]

Also, generation of direct Gaussian fields in Fourier space would be another use of GridDataSource supporting Fourier modes, i.e., #100

[nickhand]

close this for now, since most of this is addressed by #209