Moving barrier while paused

[KatieWoe]

Test device:
Dell Laptop
Operating System:
Win 10
Browser:
chrome and edge
Problem description:
For phetsims/qa#238
Moving the barrier on the slits screen produces an odd effect while the sim is paused. @arouinfar recommended that it might be good to clear the lattice in this situation .
Steps to reproduce:

1. Go to the slits screen. Any scene
2. Make sure a barrier is up
3. Start a wave and let it get across the screen
4. Pause the sim
5. While paused, move the barrier

Screenshots:

Troubleshooting information (do not edit):

Name: ‪Wave Interference‬
URL: https://phet-dev.colorado.edu/html/wave-interference/1.0.0-dev.51/phet/wave-interference_en_phet.html
Version: 1.0.0-dev.51 2018-12-12 03:47:24 UTC
Features missing: touch
Flags: pixelRatioScaling
User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36
Language: en-US
Window: 1536x732
Pixel Ratio: 2.5/1
WebGL: WebGL 1.0 (OpenGL ES 2.0 Chromium)
GLSL: WebGL GLSL ES 1.0 (OpenGL ES GLSL ES 1.0 Chromium)
Vendor: WebKit (WebKit WebGL)
Vertex: attribs: 16 varying: 30 uniform: 4096
Texture: size: 16384 imageUnits: 16 (vertex: 16, combined: 32)
Max viewport: 16384x16384
OES_texture_float: true
Dependencies JSON: {"assert":{"sha":"928741cf","branch":"master"},"axon":{"sha":"73e761ec","branch":"master"},"brand":{"sha":"1fd6682e","branch":"master"},"chipper":{"sha":"61bc739b","branch":"master"},"dot":{"sha":"b45eafe8","branch":"master"},"griddle":{"sha":"914c9215","branch":"master"},"joist":{"sha":"5a013a46","branch":"master"},"kite":{"sha":"f3e96e5b","branch":"master"},"phet-core":{"sha":"86dec8cc","branch":"master"},"phet-io":{"sha":"a267fdb1","branch":"master"},"phet-io-wrapper-classroom-activity":{"sha":"5dea7f54","branch":"master"},"phet-io-wrapper-hookes-law-energy":{"sha":"b42eaef7","branch":"master"},"phet-io-wrapper-lab-book":{"sha":"b884e9c1","branch":"master"},"phet-io-wrappers":{"sha":"d5b42848","branch":"master"},"phetcommon":{"sha":"869b2561","branch":"master"},"query-string-machine":{"sha":"e4f9e8e8","branch":"master"},"scenery":{"sha":"a18ce7f1","branch":"master"},"scenery-phet":{"sha":"56df5b6d","branch":"master"},"sherpa":{"sha":"a1f5e867","branch":"master"},"sun":{"sha":"38ed71af","branch":"master"},"tambo":{"sha":"746fbd32","branch":"master"},"tandem":{"sha":"c7a56e2e","branch":"master"},"twixt":{"sha":"e38c70ed","branch":"master"},"wave-interference":{"sha":"bfce593d","branch":"master"}}

[arouinfar]

@samreid similar to #213, moving the barrier while the sim is paused should probably clear the lattice.

[arouinfar]

This behavior looks a bit weird, but isn't technically wrong. @ariel-phet suggests we do nothing about this one, which seems reasonable to me, so closing.