general: move from SCUBA-2 to general

This primitive contained no SCUBA-2-specific code and could therefore
be used by any instrument for determining the statistics of an
image. Moving it into the general directory makes it available for all
instruments, including PICARD.

Despite the name, the primitive can be used on multi-dimensional data,
not just images.
general: new primitive created from factored-out code

The code formerly in _FIND_IMAGE_STATISTICS_ to determine the mode
from a histogram of values has been split off into a new
primitive. This helps clarify the logic in _FIND_IMAGE_STATISTICS_ and
isolates this method for debugging.
general: use a generic name for calculating SNR images

Since this primitive contained virtually no SCUBA-2-specific code, it
can be easily generalized for wider use.

The only SCUBA-2-specific element was knowledge of the WHITEN_BOX
Group uhdr entry to override the region over which statistics are
calculated. This has been removed and the relevant caller updated to
specify the region explicitly.
SCUBA2: use updated primitive name for calculating SNR images

_CALC_SCUBA2_SNR_ has been superceded by _CALC_SNR_IMAGE_