Calib: add attribute and method for SCUBA-2 error beam fraction

Simple get/set method for storing the fractional error beam derived
from SCUBA-2 calibrator observations.
SCUBA2: write error beam fraction to log file
SCUBA2: new primitive to estimate error beam fraction

This primitive uses aperture photometry to measure the fraction of the
total flux that lies outside a FWHM-sized aperture. The value is
printed to the screen by default.

Ideally this would be achieved with a single call to autophotom, but
that would require a specialist modification to the
_APERTURE_PHOTOMETRY_ primitive. Instead, use two calls and ensure
that the initial position and sky/background annulus are exactly the
same for each call.

The result is stored in the calibration system using the errfrac method.
SCUBA2: estimate the fractional error beam

Use the new _CALC_ERROR_BEAM_ primitive to estimate the error beam
fraction.
SCUBA2: minor POD update

Make sure all primitives used are listed.
SCUBA2: use pattern match for Galactic coords rather than exact string

I can never remember if the system is just GAL or GALACTIC, so cover
both bases with a pattern match rather than an exact string
comparison.