Fixed detection threshold missing in XML metadata output.

Fixed TFORM error message in fitshead.c (thanks to S.Guieu).
Fixed model-fitting error variance/covariance issue (bug in matrix inversion).
Added ELLIP1ERRMODEL_IMAGE, ELLIP2ERRMODEL_IMAGE, ELLIPCORRMODEL_IMAGE,
POLAR1ERRMODEL_IMAGE, POLAR2ERRMODEL_IMAGE, POLARCORRMODEL_IMAGE (more testing is needed and WORLD versions are not yet functional).
Narrowed the tolerance constraint for stopping windowed centering iterations.
Fixed incorrect reading of the DATE-OBS FITS keyword when EPOCH and MJD-OBS are missing.
Pushed version number to 2.11.11.