Enable metgrid to process native MPAS output files (#11)

Enable metgrid to process native MPAS output files (#11)

This merge introduces changes to the metgrid program that enable the WPS to
read and interpolate native MPAS output files, thereby allowing MPAS to provide
initial and boundary conditions for a WRF simulation. From a user perspective,
two namelist variables in the &metgrid namelist record must be set:

1. constants_name must be set to the name of the MPAS "static" file, prefixed
   with "mpas:", e.g., constants_name = "mpas:x1.40962.static.nc".

2. fg_name must provide the prefix of MPAS netCDF output files, prefixed with
   "mpas:", e.g., fg_name = "mpas:FOO"; the corresponding MPAS netCDF files
   should be named "FOO.YYYY-MM-DD_hh.nc", where YYYY, MM, DD, and hh are
   the valid time year, month, day, and hour of the data contained in the file.

Note that MPAS files should contain only one time per file. When writing output
from MPAS for use as initial and boundary conditions in WRF, the following MPAS
stream definition can serve as a template:

<stream name="wrf_ic_bc"
        type="output"
        filename_template="MPAS_15km.$Y-$M-$D_$h.nc"
        output_interval="3:00:00" >

        <var name="xtime"/>
        <var_array name="scalars"/>
        <var name="pressure"/>
        <var name="zgrid"/>
        <var name="theta"/>
        <var name="uReconstructZonal"/>
        <var name="uReconstructMeridional"/>
        <var name="u10"/>
        <var name="v10"/>
        <var name="q2"/>
        <var name="t2m"/>
        <var name="skintemp"/>
        <var name="surface_pressure"/>
        <var name="mslp"/>
        <var name="tslb"/>
        <var name="smois"/>
</stream>