Better matching between tracks, otracks

all small changes

	modified:   otracks.py
	modified:   tracks.py
Fixing (o)tracks.py to allow for non-trackable

For 'equilbrium' datasets, there are no particle trajectories, this
allows to save just the POSITIONS, not TRACKS.

Also, a nice way to load data, cdata, odata, omask from npz file.

	modified:   otracks.py
	modified:   tracks.py
Getting g(r) to work for SLR / 'equilibrium' images

 - find center of image x0, y0, since disk is not centered like in the
   high speed tif images

 - use scipy.spacial.pdist istead of manual loop :P

 - Correct pixel conversion factors for SLR image.

	modified:   correlation.py
	modified:   orientation.py