Tank contains slight concentration after restarting simulation

[bryo5363]

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 restart with other finger while water is evaporating
-Add solvent to water prior to evaporation
-Repeat evaporating solution with one finger and restarting with another finger
-The solution will have an initial concentration.

[pixelzoom]

This should probably be move to beers-law-lab.

[pixelzoom]

Moved to phetsims/beers-law-lab#73