Added list of loadable/derivable keys to ptype classes,
plus methods to load them given the corresponding quantity
name as a string.
	modified:   hdf5.py
	modified:   sph.py
loadable/derivable key names should match variable name.
	modified:   sph.py
New function load_data(), also added missing doc strings.
	modified:   hdf5.py
Removed distinction between loadable keys and derivable keys.
	modified:   hdf5.py
	modified:   sph.py
Automated memory cleanup.
	modified:   hdf5.py
Ensure loading of primary data before calculated quantities.
Also added option to not refine to highest resolution particles.
	modified:   hdf5.py
	modified:   sph.py
Override load_data for sph particles to allow for identifying sinks.
	modified:   sph.py