Fix dependence on block 0 having all PartTypes

May not be complete since line 228 still uses self._hdf[0], but I am looking at dark matter only simulations where there is only one family
gadgethdf uses gadget-units in config if no units
Fixed warnings and exception catching for gadgethdf units
Fix gadgethdf families, mass dtype, unit warnings
Fixed family error
Merge pull request #214 from alexji/master

gadgethdf5 fixes (issue #212)