Beaker contains slight concentration after 'Reset All'

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Originally reported by @bryo5363 in phetsims/joist#138.

While using the Concentration simulation or the Concentration portion of Beer's Law, it is possible to restart the simulation with an initial concentration. I am testing on iOS 8. This is a multitouch issue

Steps to reproduce
-Open simulation
-Use one finger to evaporate the water
-Press 'Reset All' button with other finger while water is evaporating
-Add solvent to water prior to evaporation
-Repeat evaporating solution with one finger and 'Reset All' with another finger
-The solution will have an initial concentration.

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I believe that in the above description, "restart" should be "reset", as in pressing the "Reset All" button. I've changed the issue title and description to reflect this.

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This is a general issue of what to do when the "Reset All" button is pressed while other interactions are in progress. I seem to recall a related issue in one of the common-code repos, but can't find it. In any case, we need to have a general discussion about this, because it affects all sims.

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I think the issue here is that sun.HSlider needs a way to cancel an interaction that is in progress. When the "Reset All" button is pressed, the dragging of the Evaporation slider (and all interactions, actually) should be ended. It currently continues. I believe there's a scenery issue that considers a more system-wide solution, which is to provide a way to end all interactions that are currently in progress.

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This is a general issue, tracked in phetsims/scenery#218. It's not appropriate to handle this specific case (and undoubtedly others) in this sim, it should be addressed generally.

I'm closing this issue as 'won't fix'. If @ariel-phet or anyone else disagrees, please reopen.