Fitting sections of files

[rwesson]

When fitting RSS and cube data, the user should be able to specify image regions to fit, rather than having to fit the entire file.

[rwesson]

Image sections can now be given, as filename.fits[x1,x2:y1,y2]. Still to be fixed is the output file naming, which starts from 1,1 regardless of the actual values of x1,y1.
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