Add testsimulations to set up simulations including integrator and system

[jchodera]

The idea comes from this thread from @jhprinz:

Another idea I had is, if instead of having test systems, we would also have test simulators which are like a test system, but without the actual topology/protein. E.g. only a simulation with a VVVRIntegrator, PME, barostat defined by some options? This way we can use it to restart a simulation without storing the system object. So a testsimulation contains a factory for a full simulation given some parameters and a topology. These could then partially be used in the testsystems? Just an idea...