Add site column to powerset during processing

[dogversioning]

To keep the data being sent from the library powerset-ish, we need to take the following actions during data upload:

• load data as a dataframe
• add a blank site column
• make a copy of the dataframe
• set the site column equal to the file path
• union the two data frames

This should probably be done at the second hop, during the latest merge section of powerset merge. last_valid data should have this operation already completed.

[dogversioning]

Addressed via #75