`ConcentrationDescriber.concentrationIncreased` Review

[zepumph]

From #171

• concentrationIncreased is boolean or null, so this line seems buggy to me:

lighterDarker: this.concentrationIncreased ? darkerString : lighterString

unless you would want "lighterString" if null or boolean. There should be an assertion making sure that it is a boolean, or documentation as to how null and false are being treated the same here.

• Some code uses this.concentrationDescriber.concentrationIncreased !== null and some code uses if ( this.concentrationIncreased ) {. Again it is not clear if here null and boolean are purposefully being linked together.

I think the more explicit you can be here, the better it will be. If ever there is a method here that shouldn't be called if concentrationIncreased is null, I think that should be asserted.

[twant]

I think this is solved by the changes in #180. @zepumph assigning over to you to review and close if you agree.