Handle different FITS extensions

[ajeb78]

GraXpert assumes that FITS files should have the extension .fits, and adds this if it is not present.

However there are in reality several FITS extensions in widespread use including .fit and .fts

This can result in a situation where if the GraXpert CLI is given an option like -output image.fit the actual saved file will be image.fit.fits (or even image.fit_background.fits for the extracted background image), which is not ideal.