Wrap the conversion of xcenter to float in a try-except block
Wrap the conversion of ycenter to float in a try-except block
Add comment explaining why {x,y}center may raise ValueError
Update docstring of QPhot.run()
Use -1 if xcenter or ycenter are an invalid floating-point number

A bug in IRAF (v.2.16.1) may cause the value of xcenter (and we assume
that potentially also ycenter) to be an invalid floating-point number,
such as '-299866.375-58'. If that is the case, use -1 as the fallback
value. This must be the least severe bug ever, considering that it
only occurs when the astronomical object is about >= 100 degrees off
the image boundaries.

A bug report has been submitted to the IRAF development team:
http://iraf.net/forum/viewtopic.php?showtopic=1468373

This merges branch 'fix-qphot-xcenter-invalid-float'