PICARD: add support for two-component PSFs

New parameter: PSF_RELAMPS - a comma-separated pair of values
indicating the relative amplitudes of the gaussians specified by
PSF_FWHM.

PSF_FWHM can now be specified as a comma-separated pair of values.

Note that if two values are given for PSF_FWHM but there is no
corresponding PSF_RELAMPS, then the standard wavelength-appropriate
values for the telescope beam will be used.
SCUBA2: add support for two-component beam

This commit contains major updates to the SCUBA2 calibration class,
including the renaming of several methods.

Whereas previously the class did not differentiate between the actual
telescope beam and a fit to the beam, methods associated with fits
have "fit" in their names.

The default two-component telescope beam properties are stored in a
hash indexed by wavelength (actually the value returned by the
subsint() method, as used by the default FCFs). The keys are FWHM1,
FWHM2, AMP1 and AMP2. The amplitudes are the relative fraction and add
to unity.

Details follow:

 * beam - now returns the hash reference containing the telescope beam
   parameters.

 * beamamps - new method to return just the relative amplitudes of the
   telescope beam.

 * beamfit (renamed from beampar) - stores/returns the fitted beam
   parameters. Note this method only stores the results for a single
   component fit.

 * fwhm - now returns an array (or array ref) with the FWHM for each
   telescope beam component.

 * fwhm_eff (renamed from fwhm) - returns the FWHM of a gaussian with
   the same area as the telescope effective beam area.

 * fwhm_fit - return the FWHM derived from the most recent fit.

 * store_beam - has been removed.

Finally, the methods have been re-grouped and reordered alphabetically.
SCUBA2: add ability to create a two-component PSF

The inputs to this primitive now permit two values to be given for the
FWHM parameter, along with corresponding relative amplitudes specified
by the RELAMPS parameter. Both are overridden by the preseence of
their respective recipe parameters.

Note that a single value for the FWHM is still accepted.

The default behaviour is now to create and use a two-component PSF.
SCUBA2: use updated method name for storing fitted beam parameters
SCUBA2: calculate ARCSEC FCFs with a default aperture of 30 arcsec

And use the FWHM of main beam component when determining the box size
for fitting to the matched-filtered map.
SCUBA2: use FWHM of main beam component as default fake source size
SCUBA2: use empirical beam area rather than assume a value