CAS-12220 : Add AWP2 support for flatPSF
AWP2 was previously gated-off because the flat PS CF kernel was being
over-written in-place by the A-term kernel, causing the flatPSF mode to
generate bit-identical PSF to the regular PSF mode.
This separates out the flat-PSF kernel from mosaic into it's own method,
and that has been hooked into AWP2.
There are still no unit tests to test this new mode, but local tests
seem to confirm that it's working (or at least not crashing...)
