[AGIPD][DARK] Correctly retrieve old constants for comparison

Closes #49

See merge request detectors/pycalibration!492

Merge branch 'fix/agipd_dark_retrieve_correct_old_constants' of ssh://git.xfel.eu:10022/detectors/pycalibration into fix/agipd_dark_retrieve_correct_old_constants

Same thing as for AGIPD.  Proposing to save this alongside the module mapping (if we're creating
YAML files per module anyway).  Note that I had to add "cap" and "qm" layers to old_const and
old_mdata; there's probably only one element in each, so the dictionary is overkill, but as we loop
over both, this would otherwise break if we ever have more than one of either...

This commit does too many things, sorry, but stuff was all tangled up:
- clean up a bit (imports, unused variables)
- map to PDU with one query instead of in loop (each query gets all anyway)
- start fetching old constants (for comparison) in the background
- add qm layer to dict holding old constants (see issue #49)

Assorted cleanup of xfel-calibrate

See merge request detectors/pycalibration!489

Install wheel package in Gitlab CI

See merge request detectors/pycalibration!493

Merge branch 'feat/agipd-darks-save-old-const-paths' into fix/agipd_dark_retrieve_correct_old_constants