SCUBA2: set parameters for fake gaussian if none given

It is assumed that if the user is calling a recipe where a fake source
is to be added to the timeseries then this primitive should always add
one, whether or not the user has specified parameters.

In the case where no FAKEMAP recipe parameters are given, the
primitive creates a fake point source using the FWHM at the current
wavelength and an amplitude of 10/50 Jy/beam at 850/450 um
respectively.
SCUBA2: document recipe parameters

Not all of the FAKEMAP recipe parameters were documented
SCUBA2: factor out some steps to other primitives

The basic map-making step is now not part of this primitive, so it
should be included in the corresponding recipe. There is an issue in
making sure that both primitives use the same config file but I'll
address that later.

The code setting the properties of the fake source has been moved in to
_FAKEMAP_STEER_.

The MAP_PSF and KEEP_SN input arguments have been removed. I can't
imagine a reason this primitive would be called if the user did not
want a map psf. KEEP_SN is effectively handled in the steering
primitive.

The Grp uhdr entry FAKESCALE is set so that subsequent calls will use
the same value for scaling the artificial gaussian.
SCUBA2: add a new line before reporting info to user for clarity
SCUBA2: check fakemap recipe parameters

The map-making step has been split into two to clarify the logic.
SCUBA2: output bounds may be obtained from LIKE, make offset an integer

The new input parameter LIKE may specify the name of a file which will
be used to define the output fakemap bounds.

Also shift the gaussian by an integer number of pixels to ensure the
peak value is preserved.
SCUBA2: recentre psf at 0,0 in pixel coords

If the artificial gaussian is to be added at an offset position, it
should be recentred at 0,0 before passing to the matched filter.

Also check the FAKESCALE uhdr value, not FAKEMAP_SCALE.

Fix a subtle bug when checking if the FAKEMAP_SCALE recipe parameter
is -1: do not store the value before checking it, since if false, the
recipe parameter will now override the value dervied from the
FAKESCALE.