SimulatorAccess for StokesMatrixFree

[gassmoeller]

Something that always bothered me about the StokesMatrixFree class was how liberally it used the pointer to simulator to access information it could have gotten from SimulatorAccess instead. This PR is a first step towards disentangling StokesMatrixFree from Simulator internals. Ultimately, I am thinking of making the solvers their independent plugin system as well. I am not sure if we fully get there, but at least with the current changes the code also looks more like a normal plugin.

This PR:

• derives StokesMatrixFree from SimulatorAccess and Plugins::InterfaceBase, to give it read access via SimulatorAccess and align its functions more with how all other plugins look like.
• changes the initialization of StokesMatrixFree to the order of functions we usually use (costructor -> initialize_simulator -> parse_parameters -> initialize).
• replaces a lot of direct accesses to the simulator (sim. ...) with simulator_acces (this->get_...)

Ideally, no functionality should change.

[gassmoeller]

@quangx and @tjhei this may affect your work on the GMG, but I think it is still worth it to make the code a bit cleaner and ultimately disentangle the solver specific code from the Simulator class.

[tjhei]

This is much nicer. Thanks.