"Hydrogen Combustion": fix "slow motion" button

Basic timeStep is 0.2, not 1.
"Hydrogen Combustion": update "spark" script + minor changes

[#57242508]

Ensure that atoms receive velocities in the opposite directions.
Slow-down model for a while.

It should be more reliable now, but still sometimes a single spark
isn't enough to start a reaction.
Merge branch 'master' into wip-java-perms