allow updating, overwriting or trimming of prep_attributes

[annakrystalli]

currently the prep_attributes() function just checks whether variableNames for a file have already been extracted and stops with an error if so. Would be good to offer options to:

• overwrite any entries associated with a file.
• append new rows for additional variableNames detected in a file.
• trim any deprecated rows for variableNames not detected in a file anymore.