Investigate custom alignment solutions for single-source exposures

[stsci-hack]

Alignment of exposures with fewer than 5(?) sources may be possible in regions where there is a nearby GAIA source. This alignment would only be an offset, no rotation or scale. Separate logic would be needed to handle this case.

This would not impact a large number of exposures, but sample datasets that could benefit include ibbs05020, ibbs05030, and ibbs05040.