Allow for the parallel simulation and comparison of trjs with unique initial conditions.

Updating my branch.  I know I will need to change some networkx stuff, and I think the result file compression handling has changed.  This will impact how the parallel trajectory pickles are recombined.

I also added an isothermal compressibility target a few months ago - it's useless (for now) as the needed sampling is on the order of 100 ns.
I don't think people should ever use the same pressure value but with different units, at least for the moment.  In the future (e.g. for the Thermo target) it might be good to either allow this or throw an exception.