Some updates to fsurdat_modifier script

[ekluzek]

There are some modifications I need to make to fsurdat_modifier tool in order to get it to work with 100% urban sites. See #1665
To do this in a general way I'm going to add some new sections to the config file for the script. I'll rename the main section from modify_inputs to modify_basic_options. The new sections will be: modify_subgrid_fractions, and modify_variable_list. modify_subgrid_fractions will allow you to override the PCT_* variables, and do error checking to make sure they are correct. modify_variable_list will allow you to modify an arbitrary list of variables that are on the file. Since it's an arbitrary list it won't have error checking, other than making sure it's a valid variable that can be set.

I am implementing in the context of a single point case, but it would work to set a regional or global case to constant values. And presumably it would work if you set the values in the config file to array values.

[ekluzek]

Another good change that could happen here is to have the list of idealized settings (when the idealized flag is set to True) in the config file rather than hardcoded in the python script. This could be it's own section and have some error checking to make sure the values set are correct.

[ekluzek]

The other part of this is to enter the filenames either on the command line or in the config file. Entering from the command line is needed to integrate with the Makefile to make surface datasets.