Only export forward slashes to tables contained in InVEST datastacks

[phargogh]

Suppose:

• We are running InVEST on Windows
• We are running a model that uses an input table T that contains a path to a file on disk
• We want to export the model's collection of inputs to a datastack

When the above conditions are met, the current output datastack contains a copy of table T, T2 where all filepaths have been rewritten to be relative to table T2. Since we are running on Windows, these paths contain backslashes representing directory separators.

It would improve compatibility (see #1479 for a related issue) when opening the datastack on a mac/linux computer if we were to save the relative filepaths in T2 exclusively using / as a directory separator. Doing so would not affect compatibility on Windows, as Windows supports using either \ or / as path separators.