Combine images with small position differences into a single SkyRegion

[mauch]

The astrometric correction made to images can cause small offsets (<10 arcsec) between the values of (CRVALx) in their FITS headers even though they were observed with the same pointing centre. Its best to combine these into a single SkyRegion to avoid having one SkyRegion per input image in the pipeline.

Also make the desired maximum offset to search in order to combine images into a single SkyRegion configurable.