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)

This is kind of overkill (also throwing processes with threads at this
particular cell), but it does in my testing save something like half a minute.

Currently, there is no way to specify most parameters when requesting darks via MyMDC - values are
just loaded from the calibration configurations repository.  To allow for using different offset
thresholds in fixed gain mode versus adaptive gain mode, adding these extra parameters is a
pragmatic if not elegant solution for now.

The relevant data is copied into memory in pretty large chunks, so the core driver is not expected
to do much for us here.  In some brief testing, using the default driver instead resulted in
approximately the same (or slightly better) runtime and significantly lower peak memory consumption.

This requires the pending MR for cal_db_interactive.  Tested with e79f2c9d (current head of the
branch of that MR).

Applying isort and nbstripout.  Changing from ipyparallel to multiprocessing - and while doing so,
simplifying the call signature of characterize_module.

Fix/pre commit isort

See merge request detectors/pycalibration!434

Currently, there is no way to specify most parameters when requesting darks via MyMDC - values are
just loaded from the calibration configurations repository.  To allow for using different offset
thresholds in fixed gain mode versus adaptive gain mode, adding these extra parameters is a
pragmatic if not elegant solution for now.