Allow specification of calibration base directory

What?

This MR allows the base directory for calibration envelopes to be specified either in the asimov ledger or in a settings file passed to asimov-gwdata.

Why?

It's possible that there may be multiple iterations of the calibration uncertainty envelopes in O4, and this should help to make us more robust against that.

Who?

This MR was prepared by @daniel-williams.

When?

This MR has not yet been scheduled for release.