Implemented spherical velocity coordinate transformations.
	modified:   pyGadget/analyze.py
	modified:   pyGadget/hdf5.py
Updated disk properties analysis with rotational velocity.
	modified:   sink.py
Added some comments.
	modified:   sink.py
Unit vector calculation works for single vector arrays.
	modified:   analyze.py
Added sink velocities to snapshot sink tracking.
	modified:   sink.py
	modified:   snapshot.py
	modified:   sph.py
Bugfix.
Convert velocities to halo/sink-centered frame.
	modified:   hdf5.py
Convert velocities to rest frame of sink for analysis.
	modified:   sink.py
Improved multitasking.
	modified:   pyGadget/sim.py
	modified:   pyGadget/snapshot.py
Corrected isothermal sound speed calculation.
	modified:   sph.py
bugfix in calculate_freefall_time()
	modified:   sph.py
Changed snapshot file location from 'sim.datapath' to 'sim.filepath'
Changed sim.datapath to represent path to auxillary data directory.
	modified:   sim.py
Replaced analyze.density_cut() with more general function data_slice()