Add aca_dark package

Functionality to include:

- Get dark cal image(s) associated with previous calibrations
- Get properties of calibrations, including all tabulated data in dark cal reports
- Get CCD dark current model for given time and temperature
- Get guide and acq success predictions for mag, time, temperature.  This is slightly off-topic but is related to overall trending.  If it seems sensible this could be moved somewhere else.

The first two could live in `dark_cal.py` and the second two in `dark_model.py`.  I plan to make an initial stub of this structure and attach code.