[WIP] Creation of an abstract base class for test systems

I've started working on the creation of an abstract base class for test systems.  This will involve the addition of methods for retrieving analytically or numerically computed properties such as
* potential energy mean and standard deviation
* volume mean and standard deviation (for constant-pressure systems)

I have also renamed `coordinates` to `positions` to match OpenMM convention.

This is a work-in-progress and not yet ready to merge in.