Move xyxymatch upstream to Astropy project?

[pllim]

At Python in Astronomy 2019, there was interest to use xyxymatch or a similar functionality (matching images without WCS using point sources in the images only) for use cases outside of STScI (e.g., for amateur astronomy or educational purpose). If the algorithm here is useful beyond STScI, perhaps it could be cleaned up and moved to Astropy project, where it could be enjoyed by a wider audience. Although, I also remember @jdavies-st saying that there is a different algorithm already out there doing the same thing?

stsci.stimage/lib/stsci/stimage/__init__.py

Line 34 in e5c8707

def xyxymatch(input,

cc @kelle and @nden

[pllim]

Currently, drizzlepac uses it and also jwst (by its dependence on tweakwcs).