Added Ulbrich and Atlas and ZA computations.
Modified the reader to improve performance.
Improvements to DropSizeDistribution main class.
Merge branch 'master' of https://github.com/josephhardinee/pyparticleprobe

Conflicts:
	pyparticleprobe/io/read_p3_2dimage_probe.py

Modifications by Joe Hardin adapted.  Eventually this may allow the merge
of pyparticleprobe and PyDisdrometer.
I had some issue with the merge and these files did
not port properly.  Manually fixed.
Update README.md

Additional description after housekeeping.