"Solid" drifts to the left on States Screen

[oliver-phet]

Start the sim and open the States screen. The atoms gradually shift to the left. This behavior isn't in the full version.

Looks like it occurs on Phase Changes as well.

[oliver-phet]

Whoops - I take it back. The full version does this too. Intentional?

[oliver-phet]

Looks like this was present in Java too: https://phet.unfuddle.com/a#/projects/9404/tickets/by_number/2057

[jbphet]

I did two things to fix this:

• added a step to the code the forms the solid that averages the velocity to zero
• added a slight concavity to the bottom of the container to push the particles back towards the center if they drift

The latter was done as part of a larger commit to address drift, see phetsims/states-of-matter@4a526e2.

So far, this seems to be working pretty well.

[jbphet]

I'm going to have to back out the concave bottom. During testing I found that this can cause side-to-side oscillations of the solid when it gets to absolute zero.

[KatieWoe]

Looks good. Closing 1.1.0-rc.1

[KatieWoe]

This also looks good in 1.1.0-rc.2 Of basics