Prevent negative DTs

[jonathanolson]

In phetsims/make-a-ten#273, @phet-steele found it looks like leaving Firefox open with the sim (and leaving overnight) will cause an incorrect ordering of dates when re-opening. This means the sim thinks it has gone back in time, stepping the simulation with a negative dt.

This can also be reproduced by stepping the system clock back in time.

Sims are not built to handle these negative dts, so we should gracefully handle incorrect ordering of dates by:

1. Don't call stepSimulation with negative dts.
2. Move back to the "old" time, so the sim will still animate after system clock changes.

Additionally, looking into whether this causes bugs for currently-available sims may be good (@ariel-phet, is this worth time, should this just be fixed in master, or should there be any maintenance releases, particularly for make-a-ten?)

[phet-steele]

This can also be reproduced by stepping the system clock back in time.

@ariel-phet it should be noted that, so far, FF is the only browser that is doing this "naturally". All browsers are affected by manually adjusting the system clock.

Also, no device other than macOS 10.12.3 has been tested at the time of this writing.

[jonathanolson]

Fix pushed to master, @phet-steele can you test make-a-ten to ensure that this prevents the bug (using the system clock?)

Fuzzing sims with possibly negative timesteps caused assertion errors in the following: atomic-interactions, energy-forms-and-changes, fluid-pressure-and-flow, gene-expression-essentials, make-a-ten, twixt demo (I'm running a longer fuzz dedicated to this to see if it pops up anything else). Note that this may not catch all bugs, because some sims may not error out (but instead behave incorrectly).

[ariel-phet]

@jonathanolson I think it would be fine to do a Make-a-Ten maintainence releases (it looks like the credits might need adjusting). But I would just fix in Master and not worry about published sims.

[phet-steele]

Chances are github thinks I'm making this comment before this issue existed (my clock is a few days behind)! EDIT: nope, github was smart.

@jonathanolson I think it would be fine to do a Make-a-Ten maintainence releases

@jonathanolson Make a Ten is fixed in current master. I tested with macOS 10.12.3 Chrome & FF, Win 10 Chrome

[jonathanolson]

Ok, with it fixed I'll plan on tracking make-a-ten maintenance release in phetsims/make-a-ten#273.