Improved modularity, using `is_main` bool

	modified:   tracks.py
Fix plot_msd, now plots correlations as plot_corr

	modified:   tracks.py
Rotation and Translation should correlate vs time.

Tau is now the variable that separates translation from orientation:

           2                   2
    < dx(t)  * dtheda(t + tau)  >

	modified:   tracks.py
Can show positive and negative correlation times

Use dtau < 0 to show both, then plf will be semilog to allow negative
values, and tau will go from -T to +T

	modified:   tracks.py
Save msd npz files with 'msd' etc, not just 'corr'

	modified:   tracks.py