Add ungrib capability to interpolate to new pressure levels (#14)

Add ungrib capability to interpolate to new pressure levels (#14)

This merge adds changes that allow for the interpolation to a set of
additional, runtime-defined pressure levels with the ungrib program.

Now, the &ungrib namelist record contains three new options:

* add_lvls: logical, whether to interpolate to additional levels
* interp_type: integer, 0=linear in pressure 1=linear in log pressure
* new_plvl: additional pressure levels (in Pa) to interpolate to

These modifications were provided by Brian Reen (US Army Research Lab) and
were based on earlier code provided to Penn State by Cindy Bruyere (NCAR).

Brian notes that:
"This capability is beneficial when quality controlling observations via
Obsgrid (e.g., for observation nudging or verification). Obsgrid interpolates
multi-level observations to the vertical levels of the coarse grid model used
to drive WRF (e.g., GFS) and then compares the interpolated observed profile to
the coarse grid model for quality control purposes. Thus, increased vertical
resolution in the coarse grid model data results in increased vertical resolution
in the interpolated observed profile used for obs nudging or verification; this
allows the observed vertical structures to be better represented in
the interpolated profile used for verification of obs nudging. The additional
vertical resolution in the coarse grid model data also improves processing of
single-level above-surface observations (e.g., aircraft obs) by providing model
data closer to the level of the aircraft ob."