SubFind HDF support

two steps to be done: 

* create a SubFindHDFSnap class that extends GadgetHDFSnap and reads in the particle data -- this is mostly completed in 5f8710c but needs to be tested
* create a SubFindHDFHaloCatalogue class that reads the FOF and subhalo group data and creates/returns the appropriate halos 

test data here: https://t.co/MjfJdcmlLP
SubFind description here: https://t.co/znFJu7F9Xz