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.

- trailing whitespace
- unused variables
- unused imports
- misc formatting

nbstripout, some flake8 (removing unused variables, imports), isort, simplifying retrieve_constants
as it does not need to work under ipyparallel

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.