Computes the MD5 hash for a file.
The hash for a data file is used for looking up data files in a unique fashion. This is of particular use for tests; a test may require a particular version of a particular file, in which case it can be accessed via hash to get the appropriate version.
Typically, if you wish to write a test that requires a particular data file, you will want to submit that file to the astropy data servers, and use e.g.
get_pkg_data_filename('hash/34c33b3eb0d56eb9462003af249eff28'), but with the hash for your file in place of the hash in the example.