library reorganization

I went through the library. I will put a first pass at some changes I think we should make to the global structure in this issue.

1. calibration.py should perform local and global metacal calibration
2. let's move the metacal class out of basic.py into a separate metacal.py and get rid of basic.py completely
3. add an ellipticity.py that switches between e1, e2, and e_t, e_x, as well as between different ellipticity conventions, this will help with handling different input catalogs
4. stats.py will handle all of the stats functions in basic.py
5. should the metacal class interact directly with catalogs? i feel like some of this should go into io.py
6. util.py should be split into binning functions and vos functions
7. survey.py has to do with the footprint, should we combine this with galaxy.py? at any rate, galaxy.py must be renamed. also, the conversion between fwhm and sigma belongs in stats
8. it makes sense to group all cosmology things in one place, but i think there needs to be a further separation of the theory and data functionality.
9. galaxy cluster functions needs to be together
10. catalog.py again is a mix of stats and io, though it does perform a number of initial sanity checks. maybe we should rename it to cat_quick_check or something along those lines?

comments welcome- I'm sure I didn't consider every use case, so there are probably adaptations of the above that will make more sense.

Start by mapping imports and uses across calibration.py, basic.py, metacal.py, ellipticity.py, stats.py, io.py, util.py, survey.py, galaxy.py, catalog.py, and the cosmology and galaxy-cluster code. Resolve the open structural questions with maintainers, then define the agreed module layout and verify that existing functionality remains available after the reorganization.