Created first draft of abstract base class.
Updated abstract base class implementation with NotImplementedException and potential energy mean and standard deviation retrieval methods.
Abstract base class analytical property computation methods now return None instead of NotImplementedException.
All test systems now subclass TestSystem abstract base class instead of object.
Added two additional properties (volume mean and stddev) to abstract base class.
Renamed 'Stddev' to 'StandardDeviation' in method names.
Fixed a typo in docstrings for units of potential energy.
More changes to abstract base class, and implementation of mean and standard deviation of potential energy for harmonic oscillator.
Reworked abstract base class to use @property variables, but not sure if this is the best way to go.
Major reworking of how analytical properties are computed.
Added ideal gas test from old pyopenmm distribution.
Added comparison test of system energies across platforms.
Added pyopenmm code for testing energies with tabulated reference values to see if anything has changed.
These tabulated values should be moved into the test_systems.py file as properties.
Added script to test custom integrators.
Added base class function to serialize test system to XML.
Updated test_platforms to emit serialized XML files when failures are detected to aid in debugging.
Added additional testing to break down discrepancies between platforms by Force object when platforms disagree.
Added in option to specify whether analytical dispersion correction should be used for AlanineDipeptideExplicit.
Reorganized order in which information is displayed in test_platforms.py
Merge pull request #5 from jchodera/base-test-class

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