Finding pols in file names

[nicholebarry]

Line 114 in fhd_file_setup looks for the polarization names in the file name:
pol_exist = stregex(datafile, '[xy][xy]', /boolean, /fold_case)

This has unintended behaviour if a pol name exists in the file name within a word. For example, the polarization xy will be pulled from file names that have the word galaxy even if the xy polarization does not exist.