Document and clarify the Propagator and EigenStepper code (was ACTS-336)

Original author Hadrien Benjamin Grasland @hgraslan

The RungeKuttaEngine has reached an extreme level of technical debt that makes code maintenance and evolution almost impossible. One purpose of its proposed long-term replacement by the Propagator-based architecture is to make the ACTS propagation code understandable by human beings again. However, this goal will only be achieved if we avoid making the same mistakes twice by making sure that the rewrite is as cleanly written and well-documented as we would have liked the original to be.