Update compute_nu_at_OBL_depth

Computation to mimic how POP computed diffusivity at OBL depth was incorrect,
moving towards correct computation (still testing) required a different
interface because I wasn't sending everything that was needed. I think we
should have two different interfaces for compute_nu_at_OBL_depth, and the
check to see if you are using POP interpolation should be done outside the
routine; once this is working I'll move in that direction.
More bugfixes for compute_nu_at_OBL_depth()

Biggest issue was that I treating layer_nu(1) as a cell-centered value rather
than an interface value when interpolating; also had the sign wrong on a
couple of intermediary derivatives.
compute_nu_at_OBL_depth works, coeffs_kpp doesn't

Small tweaks to cvmix_coeffs_kpp_low, still not consistent with POP. However,
compute_nu_at_OBL_depth does work and I want to save my progress before
updating the interface.
Rename clean up interface for compute_nu_OBL_depth

If you want to ocmpute diffusivity at the OBL depth with standard polynomial
interpolation, don't use compute_nu_OBL_depth... this routine is specifically
meant to do the interpolation / weighted averaging laid out in Appendix D of
LMD94.

Note that this commit just comments out calls from cvmix_coeffs_kpp_low, next
commit will have that cleaned up. I may need to re-write cvmix_coeffs_kpp_low
now that I have all the calls laid out correctly in POP...