Finished Pacanowski-Philander reg test

Parameters are all set correctly to reproduce Figure 4 from the Pacanowski and
Philander 1981 paper (by running plot_PP_diff_coeffs.ncl). I needed to tweak a
few parameters in cvmix_shear_drv.F90, and I also output Mdiff instead of Tdiff
to be consistent with the paper (they plot nu, not kappa)... this meant
changing the ncdump line in shear-test.sh. Lastly, in the shear reg_test
directory, I renamed plot_diff_coeffs.ncl to plot_LMD_diff_coeffs.ncl because
there are now two plotting scripts available.