+ Make `GetIndex()` work consistently whether `IndexedLookup`
  is true or false.
+ Make `GetIndex()` and `MapValue()` properly handle NaN values
  when `IndexedLookup` is false.
+ Return a `vtkIdType` for the number of annotated values.
  (The previous `vtkVariant` was a cut-and-paste error.)

Change-Id: I4586ab117200cca73b021ce0d01782a7da958b45