Second minor update to v0.60 (v0.62)

Updates to shared code:

1. Treat RICH < 0 like RICH = 0 for Pacanowski-Philander mixing (we already do this in the LMD shear mixing).
2. Remove "no_diff" option for computing shear mixing coefficients

Updates to reg tests:

1. Add PP test (recreate Figure 4 from their paper)
2. Lots of clean up to shear testing directory and subroutine to account for additional test
3. Improve cvmix_io routine for writing multiple columns of data at once
4. A little bit of clean up in background and double diffusion driver routines