Additional memory usage optimization.
	modified:   analyze.py
Simplified and streamlined.  Instead of having separate functions for
'density' and 'structure' visualization, now have single function
'density_projection' with different viewing/centering options obtained
by calling 'set_viewpoint'
	modified:   visualize.py
Change default resolution to pps=500 instead of 1000.
	modified:   visualize.py
Changed some variable names for conciseness.
	modified:   visualize.py