Add new capabilities from Kevin Wilson to handle an optional

Add new capabilities from Kevin Wilson to handle an optional
relational symbol, < or >, in the specification of interpolation
masks in METGRID. For example, rather than specifying that no
values where the LANDSEA field is 0 should be used with

   interp_mask = LANDSEA(0)

it is now possible to, e.g., specify that no values should be
used when the LANDSEA field is less than 0.5 with

   interp_mask = LANDSEA(<0.5)

This new capability also works with interp_land_mask and
interp_water_mask. Currently, either '<', '>', or no symbol
at all (implying strict equality relation) are supported.

M    geogrid/src/interp_module.F
M    metgrid/src/interp_option_module.F
M    metgrid/src/process_domain_module.F

git-svn-id: https://svn-wrf-wps.cgd.ucar.edu/trunk@591 86b71a92-4018-0410-97f8-d555beccfc3a