Added VolumeCodec and test, now used by VolumeClient and the mock server.
Also, added a modify function to VolumeClient (untested).
h5mockserver: Support POST requests for subvolume modification (with test).
Volume mimetype is application/octet-stream, defined in volume_codec.
mock server: Propagate errors as exceptions.  For internal errors, send a traceback in the response.
client: Defined a custom exception type.
mock server: Always print server exit message for command-line runs.