Make testsystems file

[proteneer]

Per #337

we should find all instances where we load in the hif2a set and a pair of ligands and have them load from a common test system function.

files of interest

examples/rbfe_single.py
tests/test_free_energy.py
tests/test_topology.py
tests/benchmark.py
in a separate PR soon to come
tests/test_minimizer.py

[maxentile]

The singleton design in https://github.com/proteneer/timemachine/blob/12bcc2e8d590ea6eded7124aa316ce14a6bfc703/testsystems/relative.py has caused problems, e.g. when an application mutates the test system. Modeling this instead on the testsystems modules in openmmtools, pymbar, or msmbuilder would be a good idea.

[proteneer]

I think this is mostly done.