SCUBA2: write log file + store noise mosaics for later use

This primitive now writes out a customized log file summarizing the
results of the QA calculations in a way that allows easy plotting with
Topcat. The log file is named according to the current wavelength so
450 and 840 data are logged separately.

The noise mosaics are now stored and tagged with "scinoi" so they can
be retrieved at a later date and turned into a 3-d cube.
SCUBA2: new primitive to stack images into a 3-d cube

Thsi primitive will retrieve all images tagged with a given tag, along
with any specified NDF extensions and stack them using stackframes to
create a cube ordered by MJD.

Note that currently all the images must have the same dimensions.
SCUBA2: add call to _STACK_FRAMES_ to create noise cube