Support compressed (gzipped) XML data files

[brynpickering]

Often MATSim input/output XML files are compressed. I just tried genet generate-standard-outputs on the command line with a gzipped network XML and it failed. So I'm assuming GeNet can't handle compressed files.

My initial stab would be to load network files into memory using gzip in gent.input.matsim_reader.read_network if they end in .gz. Similarly for schedule and vehicles (less likely to be compressed?)