Further improve CompositeSimulator

[paul-buerkner]

I think the CompositeSimulator has an excellent design. To further improve user-friendliness I suggest the following two changes:

(1) Rename to SequentialSimulator. I think this is a more descriptive name.

(2) Merge CompositeSimulator and CompositeLambdaSimulator into one function (SequentialSimulator), which takes a list(?) of objects, each of which is coercible to a Simulator object. First, we go over the list of objects, transform each of them to a Simulator (if it is not already one), and then execute the code currently implemented in CompositeSimulator. This way, users have a single interface for building sequential simulators, and can use any object which we deem valid to serve as a simulator.

[LarsKue]

To accommodate this, we will add a dispatch utility for simulators.