Store color strings on Solute.js instead of SoluteDescriber.getCurrentColor()

[zepumph]

From https://github.com/phetsims/molarity/issues/171

It feels much more robust to store color strings in the model, rather than have a switch case in a describer. I also feel the same way about the lowercase versions of the solute names.

This feels better to me for a few reasons:

• The switch case obfuscates the additional logic needed to support adding/removing Solutes. You may remove something from the model, and not know to change the switch case to keep things in sync
• This parallels the way everything else is created in this sim, in which MolarityModel.solutes is the kingpin, and everything get's its solute data from that list. It would be nice if the guts of where MolarityModel.solutes is declared was the only spot where solute-specific data was needed to be given.

[zepumph]

I implemented this above. During it I found two things that I would still like to clean up before review:

• rename getCurrentSolute because it returns a string.

• 120 char clip on MolarityModel that is a bit strange.

[zepumph]

Alright, cleanup is done now. @twant please review. Especially about the placement of these strings in the model, and if you think the new StringCasingPair data structure makes sense (and has a good name).

Thanks!

[twant]

@zepumph I like this a lot, particularly adding the lowercase names and colors to the model. StringCasingPair is nice, and I want to look at ways this could be used in other places where we have capitalized and lowercase string versions. I changed a few string names in this in a commit related to #172 to make sure we're consistent in identifying strings as lowercase or capitalized in the variable names (just as a flag that two versions exist, and for consistency). Thanks for doing this! Closing.