Apply facet offsets to images during mosaicing

[twshimwell]

Related to issue #351 we need to redo the astrometric corrections as previously poor quality corrections have been applied. Going back to the uv-data for redoing the corrections is prohibitively expensive and thus we need to do it on the images.

An example of applying shifts to an image and having some padding around the facets to avoid creating gaps is in the offsets_compact branch script https://github.com/mhardcastle/ddf-pipeline/blob/offsets_compact/scripts/apply-astrometric-shifts.py. However, we intend to incorporate this shifting into the mosaicing allowing us to avoid doing two regridding steps on the images. So mosaicing needs to correct the astrometry (based on output of offsets.py - #351) of each facet whilst doing the reprojection.

Might as well do this for both low and high resolution images.