Option to select only thermodynamic levels

[neishm]

For some FSTD files (such as GEM output), there's a staggered vertical coordinate with "thermodynamic" and "momentum" levels. Usually I only want to include fields that are defined on thermodynamic levels, but also GZ. Since GZ includes both sets of levels, it's giving me 2 vertical dimensions in my netCDF file, and there's currently no easy way to filter the momentum levels out.

Proposed solution is to add a --thermodynamic-levels option to explicitly filter out those unwanted levels. Possibly could also check if this particular use case is being triggered, and recommend that option to the user for re-running the converter?