Add support for single-column PP test

Add another namelist to cvmix_shear_drv.F90, update driver to print out
diffusivities from both LMD single column (as before) and new PP single column
test. Also updated plot_diff_coeffs.ncl to reflect new file name... next commit
will be to rename plot_diff_coeffs.ncl to specify it's for the LMD plot (and
create a new script for the PP plot)