Cannot read UF files where number of fields changes between rays

[jjhelmus]

When the number (or order) of the fields changes between rays in a UF files, Py-ART cannot read these files correctly as the get_field_data method of the UFFile class assumes that the number of order of the fields is identical between all rays, see lines 184-189 in the uffile.py modules.

Thanks to Dustin Conrad for informing us of this bug and providing a sample file which exhibits this behavior.