DRY up selector defaulting

[rod-glover]

Components VariableSelector, TimePeriodSelector, SeasonSelector, and RegionSelector all do generically the same thing to select a default value. However they do this same thing somewhat differently in each case, and this is unnecessarily complicated and confusing.

The generic thing is that they use replaceInvalidValue to select one of the selector's options (derived internally in each selector) when there is an invalid value (usually undefined) for the selector. This makes defaulting easy and quick. The option matching function should be abstracted out to a single, common function, which might profitably be migrated back into component pcic-react-components.