Enable saving and restoring full simulation state

[kyllingstad]

This closes #768. Some changes may warrant a bit of extra explanation:

execution and algorithm: I have taken one step towards prohibiting adding/removing sub-simulators after the co-simulation has begun, as decided in #771. In particular, I've added the execution::initialize() function, which marks the point where such changes are no longer allowed. (This is a backwards-compatible change, because it gets automatically called by execution::step() if it hasn't been called manually.)

slave_simulator: The changes in slave_simulator.cpp really ought to have been included in #769. Unfortunately, I didn't realise they were necessary before it was all put into the context of saving algorithm and execution state. Basically, I thought I could get away with just saving each FMU's internal state, but it turns out that we also need to save the slave_simulator "get" and "set" caches.

(For those interested in the nitty-gritties, this is due to some subtleties regarding exactly when the cache values are set in the course of a co-simulation, relative to when the values are passed to the FMU. At the end of an "algorithm step", the "set cache" is out of sync with the FMUs input variables, and won't be synced before the next step. Simply performing an additional sync prior to saving the state is not sufficient, because that could have an effect on the FMUs output variables, thus invalidating the "get cache". That could in principle be updated too, but then the slave_simulator is in a whole different state from where it was when we started to save the state.)