SCUBA2: update POD to reflect previous commit
SCUBA2: just grab median directly from the primitive hash

No need to store the results from the statistics calculation - just
retrieve the median as needed from the results in the primitive hash.
SCUBA2: store science duration in uhdr

The mapstats log file needs the duration of the science part of the
observation so store it here for later retrieval.
SCUAB2: new primitive to write out summary logfile

This primitive calculates and writes out a series of values derived
from the reduced image, including the RMS noise, NEFD and exposure
time. Note that it only works on Frames and should not be used to
analyze Group files.