Several changes related to the multisystem propagators:
some cleaning
add an algorithm class that implements the concept of algorithms as lists of algorithmic operations.
use the new classes for the propagators
move the description of the algorithmic operations specific to a given propagator to the corresponding module
The long term idea is that each system should have an algorithm that is executed during the calculation. This might be something different than a TD propagator, like an energy minimization. Before we get there, more changes are required to the framework, but this will be done over several merge requests.
