Add framework for reading data from file to pass to physics suites

New namelist variable format:

>` < stdname >_file = "< filename >"`
> `< stdname >_frequency = "init"`
> `< stdname >_file_varnames = "< varname1 >,< varname2 >"`
> `< stdname >_interpolate = .false.`

1. Add metadata file based on scheme's namelist variables (will be auto-generated)
2. Read namelist variables (in gw namelist reader that will one day be auto-generated) & register info from namelist in hash table (declare variable as initialized)
3. Use _frequency to check if it's time to read the data. If so, find the variable in the file using _file_varnames list.

(driving force for this framework: need orographic data for gravity waves scheme)