Handle saving datastack with relative paths, across drives/volumes

[davemfish]

When creating a datastack JSON file, users can choose relative or absolute paths for files. Relative paths are relative to the save location of the JSON file. So when that is on a different drive from one of the files in the datastack, it raises,

Traceback (most recent call last):
  File "flask\app.py", line 1473, in wsgi_app
  File "flask\app.py", line 882, in full_dispatch_request
  File "flask_cors\extension.py", line 165, in wrapped_function
  File "flask\app.py", line 880, in full_dispatch_request
  File "flask\app.py", line 865, in dispatch_request
  File "natcap\invest\ui_server.py", line 186, in write_parameter_set_file
  File "natcap\invest\datastack.py", line 568, in build_parameter_set
  File "natcap\invest\datastack.py", line 541, in _recurse
  File "natcap\invest\datastack.py", line 541, in <genexpr>
  File "natcap\invest\datastack.py", line 555, in _recurse
  File "<frozen ntpath>", line 791, in relpath
ValueError: path is on mount 'C:', start on mount 'D:'

We need to handle this more gracefully, at least when the Flask application is the caller, because we don't want to crash the app. I think it's okay for datastack.py to raise this exception.

We could refuse to create the datastack altogether. Or we could revert to using absolute paths.