created metrics package

split things in metrics.py into several pieces, in a new package.
metrics does measurements
created plotter package

moved code to create plots into a new package, and split out each plot
type into its own file
created n_body package

moved n_bodies code into a package to be tidy
created util package

includes a few miscellaneous functions, including code to randomly
generate initial conditions for homogeneous sphere and plummer sphere
created unittests
made it so it works now

also now runs for varying softening
simple script to instantiate a plummer sphere and watch evolution

only conserves energy at optimal softening
added code to find 1/4,1/2,3/4,1 mass radius
change to virial units
don't sync outputs

clutters up git status
explore effects of softening on error at different radiuses

implement Hu Zhan's (2006ApJ...639..617Z) method of finding optimal
softening