Generic algorithms

Several things that we need to make allow for more than propagators in the multisystem framework and, hopefully, better structure the code:

• Introduce a propagator factory !1909 (merged)
• Add an abstract algorithm factory and use that in system_t. This means the actual factory must be passed as argument to the relevant routine (system_propagator_init?) !1909 (merged)
• Replace propagator with something like algo or algorithm in variable and function names !2021 (merged)
• Swap the dependency between system_t/multisystem_t and propagator_t. This will enable/require moving some code from system_t/multisystem_t to propagator_t !2021 (merged)
• Replace the pointer to a propagator_t in system_t to be of type algorithm_t. !2021 (merged)
• Introduce generic iteration counters !2242 (merged)

Other related MRs: !2185 (merged) !2029 (merged) !1991 (merged) !1990 (merged)