analyze.py: new method orbital_frequency().
analyze.py: allow chaining of operations without recalculating.
analyze.py: orbital_frequency() misnamed. Changed to angular_velocity().
sink.py: added an update_index() method to Sink class.
snapshot.py: vxyz --> uvw
sink.py: disk_properties() revamped to use keplerian rotation selected disk.
Updated SQL queries.
Raise IndexError if trying to analyze disk for a non(yet)existent sink.
sink.py: Added ability to pass in already loaded snapshot to disk_properties()
sink.py: Close snapshot files in disk_properties() after loading data.
sink.py: cleanup snapshot to manage memory usage in disk_properties()
Reverted last three commits.  All errors in judgement.
sink.py: use kwargs with AccretionDisk.load()
Use more particles and lower densities to find center.
	modified:   analyze.py
New scripts.
	new file:   accretion_disks.py
	new file:   sinkhistory.py
Updated script.
	modified:   analyze_halos.py
Merge branch 'dev' of github.com:hummel/pyGadget into dev
multiplot.py: Updated electron fraction plot limits.
multiplot.py: Updated electron fraction plot limits.
Updated sinkhistory.py to index by time.