New function pair_indices

Replaces np.triu_indices for speedup and easier understanding

	modified:   correlation.py
Don't load stuff on import

Added a few if name==main lines.

	modified:   otracks.py
	modified:   tracks.py
Split pair_angle_corr into two functions

One returns the global order parameter (OP),
the other returns the correlation function

This allows me to do less work if all I want is the OP.

	modified:   correlation.py
New function structure_factor

Doesn't work well: slow and inaccurate.

	modified:   correlation.py
New function distribution

distribution(positions, ...) returns the full 2d distribution function
g(x, y)

to avoid confusion, renamed pair_corr to radial_distribution

Also renamed global_particle_orientation to orient_op
(should have been in earlier commit :P )

	modified:   correlation.py
Notebook now has full distribution function

Includes averaging over rectified g(x,y),
(with rotation by the phase of Psi_4)

	modified:   orientation.ipynb