Create a tool that generates a table of settings values

[mnlevy1981]

@matt-long asked if there was an easy way to produce a table of default settings values for a paper (so output would be in LaTeX) and also for the web documentation (so reStructuredText or maybe just re-use the LaTeX?). I think the best approach would be a new python script that would populate a pandas dataframe and use the to_latex() function to produce the desired output; input would be a YAML file with the following information

• Requested columns in the table (match keys in the settings YAML file)
• Output format
• List of variables to include in the table
• For each variable, a way to override defaults for things like variable name, longname, units, etc (to allow for better formatting)

We can take advantage of the fact that MARBL_tools/MARBL_generate_settings_file.py already does a lot of the heavy-lifting we need to make sure we get the right default value for each variable.

[mnlevy1981]

Closing this -- #346 is the only update the belongs in this repo, and an example of how to use it can be found in https://github.com/marbl-ecosys/cesm2-marbl/pull/37