CAS-12220 : new psfmode argument in tclean.xml + cleanup
Add a new "psfmode" argument within tclean that triggers the gridders to
use the flatPSF path, and complete the pipes all the way down into
SynthesisImager.
The old tclean.py hack has been removed in favour of the C++
code-changes.
CAS-12220 : Fixes for AWP flatPSF path
AWP was crashing with the previous flatPSF implementation,
this cleans up some old code and fixes the flatPSF path.
AWP2 is still unsupported with these fixes, that will be looked at later
on. It raises an error if flatpsf is used in conjuction with AWP2.
