Moved coordinate rotation operations from visualize.py to coordinates.py
cleanup
analyze.py: added routines for finding angular momentum, moment of inertia,
	    and axis and angle required for rotation to face-on view.
coordinates.py: rotate about arbitrary axis (unit vector) by arbitrary angle.
visualize.py: added view option 'face' to set_view()
hdf5.py: orient_box() set up for 'face' view (need density loaded)
sink.py: disk_properties() default view now 'face'
hdf5.py: bugfix
analyze.py: split angular_momentum() into two functions to allow access to
	    individual particle's angular momentum.