Mostly correction in dynamics_engine.

Previously, we didn't check whether the first snapshot already hit the stop
condition.

Also resurrected fix_shooting_mover.ipynb, and some visual cleanup in
pathmover.py.
Now it works! Next undo any unneeded changes.
Undid some unneccessary changes.
Removing the fix_shooting_mover.ipynb.

Useful for debugging, shouldn't be needed now.