Draft: RFC: New external engine for AMS

This is a new external engine interface making PyRETIS work with the Amsterdam Modeling Suite.

This series is not supposed to be merged directly just yet.

Instead, the "RFC" commits are meant to start discussion on how best to make PyRETIS core ready to work with the AMS engine. Some of those commits are nearly good to go (fixes to what seem to be clear bugs), others are horrible hacks that should probably be replaced by something more sustainable.

Basically, unlike other engines external engines, AMS keeps running in the background while PyRETIS asks it to perform multiple simulations, so we can't just rely on shuffling files around but have to keep AMS informed of any copies/renames/deletions of individual configurations.

(The series doesn't apply cleanly to current develop branch due to some methods that got renamed in the meantime, I will sort that out later.)