Split diff_iface into Tdiff_iface and Sdiff_iface, renamed visc_iface to Mdiff_iface (should maybe be Udiff_iface or Vdiff_iface?)

The library (libcvmix.a) builds, but the stand-alone driver doesn't (yet) so I haven't actually tested these changes.
Continued update to account for new variable names; stand-alone driver now builds. Will run some tests and update output scripts as needed.
updates to get BL stand-alone driver to work correctly
updated double_diff test
updating shear test
Updates to kpp test, fixed error in output variable name in double diff driver
Update tidal test script