Adding file_name to data dictionary

[wavingtowaves]

Submitter: @robcrystalornelas and @tvelliquette

I suggest the following changes:
In a future revision to the FLMD reporting format and data dictionary we may want to consider adding a file_name column to the data dictionary. This is based on how USGS Sciencebase formats its Data Descriptions files: https://www.sciencebase.gov/catalog/item/5fff2994d34e592d8671ecdb

This would help make a more direct connection between FLMD, data dictionaries and the data files themselves.