Add facility to edit pfsConfigs at read time

Every couple of runs we find ourselves needing to edit pfsConfig files that have already been written. Instead of modifying the raw data, we should add a facility similar to the header patching code. There's a clear place to plug it in (the formatter), and I would propose that the system consists of a function that is given a pfsConfig and returns a (potentially modified) pfsConfig (i.e., the library of required changes to pfsConfig files consists of code, rather than JSON files or similar).