Add convenience methods or properties to testsystems to get sets of systems for testing

[jchodera]

It would be useful if we could easily query the openmmtools.testsystems module for useful subsets of systems, such as:

• all testsystems of any kind
• main test representatives (e.g. WaterBox, but not DischargedWaterBox)
• all alchemical test systems
• weird corner cases that are non representative of real use cases
• systems that are typical of real use cases
• systems that will be "fast" or "easily computable"
• systems with analytical properties

There's a lot of stuff in there now, but not all of it is applicable to every kind of test battery.

[kyleabeauchamp]

I think this might be done easily by one of two methods:

1. Add @property decorators for various attribute classes
2. Possibly use Mixins to provide these property decorators
3. Continue to search subclasses via find_subclasses() but condition on the desired property