Extended spreadsheet_write to support returning the output table

rather than having to write it to an output file every time.
The kwarg I added is 'nowrite'.
Wrote spreadsheet_write_efficient.

It finally generalizes the ridiculously crufty "variability script" and "variability script orion" type stuff (with the by-hand retuning of kwargs every time, etc).

I think it works.
Extended base_lookup to accept any sourceid offset you want.

Defaults to the old hardcoded value for backwards-compatibility. Zero works pretty well as an alternative.
Removed a commented-out print/debug statement.
Compressed what gets printed in spreadsheet_write_efficient.

Someday I'll add in a verbose kwarg when I figure out how to make it play nice with the **kwargs that are already sitting in there.

Also I'll figure out how to print multiple things to a line and watch stdout update live rather than in a batch all at the end.