Code review of vtkVariant and vtkScalarsToColors

- added range checking to GetAnnotation() for consistency with GetAnnotatedValue()
- improved various comments as I learned the details of these classes
- changed vtkVariant's Valid ivar to bool, the related methods were already using bool
- changed vtkVariant's Type ivar to unsigned int, the related methods were already using unsigned int
- use VTK_VOID instead of a raw 0 for vtkVariant's Type
- make use of constexpr in a few places
- always use .0 so that doubles look like doubles and not ints
- prefer preincrement in for loop
- instead of guarding possible null pointer deref by checking Valid check the pointer itself.