Fix auto ids on timestamp fields

LPA Excel files have a datetime column as _timestamp. It blows up when joining as string. This should address it.

Some more context:

We're now parsing timestamps from the excel docs as iso strings and letting the record class turn them into datetime objects as they his the database layer. We'll probably need to revisit this later.

We also discard blank lines from the excel sheet as before they were being stored with an id and blank fields.

@gtrogers and @phss