Drizzlepac/HAP: Determine new fit metrics to add to header for rel fit.

[stscijgbot-hstdp]

Issue HLA-1332 was created on JIRA by Steve Goldman:

When relative fitting, the fit metrics (nmatches, fit_RA, and fit DEC) will always be the same for all of the images in an association. We need to save different metrics (e.g. rmse, mae, std, resids) instead to the headers in order to save information about individual fits.