Add pickling support to C++ solver dummy

[IshaanDesai]

This PR adds functionality to allow for pickling and unpickling of the C++ micro simulation code in the C++ solver dummy. This is based on the pickling support documentation of pybind11.

[erikscheurer]

Some questions from my side, otherwise it looks good.
I think it would be very useful for this PR to be also merged into #29, because if we have a get and set state method, implementing the __deepcopy__ function would be as easy as saying, create a new instance with self._sim_id, get_state from self and set_state of the new instance. This could then be part of the c++ dummy and used without modification by the user or could even be ignored as written in the pybind deepcopy documentation.
If we write in the documentation that the simulation state should be fully contained in the set_state method, it is more clear that not every variable of the object has to be copied as I did in the deepcopy implementation with the checkpoint as well, but only the necessary ones.

[erikscheurer]

I get an invalid state error running:

import pickle
import micro_dummy
sim = micro_dummy.MicroSimulation(2)
pickle.loads(pickle.dumps(sim))

I think this is because py::make_tuple is called on a std::tuple returning a tuple of a tuple ie length 1.
Moving the make_tuple call to the get_state function seemed to solve the issue for me, but I don't know if this is the intended definition of the getState function.