RT: added photon groups
After a long hiatus, here's the news:
- renamed M1closure files/dirs to GEAR to avoid ambiguities with other schemes
- added RT_NGROUPS definition to configure.ac for GEAR scheme
- added function (prototypes) to read in RT particle/star data
- added writing photon groups to snapshots in GEAR scheme
- started provisional RT documentation
I've attached a sample snapshot output in case anybody wants to check the resulting outputs.output_0002.hdf5