Generalize FITS file reading.

[KshitijT]

Currently fileio.py is a bit hardcoded; for a FITS file with say, NAXIS=2, readFITS crashes. Perhaps it can be modified to take number of axis into account while reading the data ?

[griffinfoster]

There is a small fix to handle different image dimensions, but it is not the ideal solution. The FITS reader and WCS generator is a hack. This should be improved, see the aplpy FITSFigure() function.

Currently for FITS image cubes with polarization and frequency axes, only the first polarization and first frequency is sliced out. Need to include a polarization option, and frequency selector/averager.