gadgethdf5 fixes (issue #212)

Fixes dependency on all PartType being in the 0th hdf file

Please note that there is one thing I did not fix because I only have one family in my files and I wasn't sure how to fix it in that case.
I also added another commit which adds the option of using gadget-units instead of storing the units in the HDF5 files.

A temporary test file is here:
https://www.dropbox.com/sh/otg8a26qnkuvt4a/AACFGcJfLwACEGgy3P2sxaKwa?dl=0
Note that since the positions in this test file are stored in 32 bit floats, I had to edit DummyHDFData to use dtype np.float32 for producing the kdtree.
"name" will loop through PartType1, PartType2, etc. If this particular hdf doesn't have that parttype, it will error without this exception handler.

I'm wasn't sure if any other situation could arise, so I decided to only ignore this case.