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~~ DONE
* in 13bd72a added support for loading subhalos of fof groups, e.g. ``h[0].sub[0]``

test data here: https://t.co/MjfJdcmlLP
SubFind description here: https://t.co/znFJu7F9Xz
yes but I like this hack :) I wasn't sure if @astroduff wanted us to include his code in pynbody...