[Dart-users] update to wrf/dart model_mod

[Nancy Collins]

if you aren't a wrf/dart user, you can discard this note.

last evening i committed a minor update to the wrf model_mod
which makes the code to handle pressure and surface pressure
interpolation more like the other obs which have 3d and 2d versions.
(thanks to ryan torn for this code.)

i also fixed some holes in the new error checking for which kinds
are allowed to be interpolated based on what is in your namelist.

there should be no change in the results from this update, but
it should give fewer spurious errors about combinations of fields
in the state vector which are, in fact, ok.

nancy