potential fix for problem with subviews of arrays referring to the global sim instead of the subsim
Merge pull request #152 from pynbody/fix_array_family

make arrays from subviews refer to the subview instead of global snapshot
added set_units_system function to SimSnap that allows one to change the automatically loaded unit system
improvements to the set_units_system function -- can now call it to set up the units for a snapshot made with pynbody.snapshot._new()
Merge branch 'master' into gadget_hdf_fix