rename FilesFits to FPCommandOutput, refs #5535

In future versions Archivematica will allow alternate characterization
paths; this may mean that files will be characterized by tools that
aren't FITS, or will be characterized by more than one tool. This new
table allows multiple rows per file, and references FPR's fprules
table in order to allow referencing the rule's "purpose".

In the future this table may be used to store the output of other
FP commands too, not just characterization, and so selecting WHERE
purpose = 'characterize' will be important.