fix for #156 -- support loading DM only files for gadgethdf

fixed a few things: 

* check which particle types are actually present https://github.com/pynbody/pynbody/blob/gadget_hdf_fix/pynbody/gadgethdf.py#L107-L113

* fixed the parameter and unit decorators to fail gracefully if certain things aren't there
* recognize to use default units when no unit information is present and issue a warning (as per what is done for the tipsy format) 

still needs fixing: 

* ~~in the file given to me by Brendan, it seems that the velocities array is not being read properly, i.e. `s['vel']` doesn't find the 'Velocity' array~~ (fixed in 
3eabb766fa4fae327143e0aaeab604c5bf74cc93)

need to check: 

* does this break outputs with dm+stars+gas? I don't have one to test