expand shmdir functionality to all DiskReporters

Make the optional shmdir kwarg influence all DiskReporters, not just GWFSeriesReporter. Similarly, have them return a list of paths to which they wrote, not just a single path. Calling functions should then report the full list of locations to which the data was written, probably with something like: ', '.join(paths).

We may need to make other Reporters also return lists (instead of single paths) to which they wrote material to maintain a uniform API across all Reporters.