`tables` is now a requirement

The tables package is now a requirement due to changes in calibration. Can we make this an optional dependency?

cc @ethan.payne @moritz.huebner @colm.talbot

@gregory.ashton I guess that since it is standard for GW-required imports to be listed in optional_requirements.txt, I think we can do the same for tables.

I agree

!930 (merged)

mentioned in merge request !930 (merged)

closed with merge request !930 (merged)

mentioned in commit 125c2660