Add support for selection of an array and/or array component for
opacity mapping that is distinct from the array used for coloring
mapping.

The volume representations have been augmented with three new
properties:

* UseSeparateOpacityArray - indicate when a separate opacity array
  is to be used.
* OpacityArrayName - the array to use for opacity mapping
* OpacityArrayFieldAssociation - the field association of the array
* OpacityArrayComponent - the component of the opacity array to use
  for opacity mapping

When enabled, the volume representations assemble a two-component data
array from the color array and selected opacity array to hand to the
volume mappers, some of which support opacity mapping via a
two-component arrays. The volume representations were refactored to
move common code into a base class and take advantage of modern C++
features.

Finally, transfer function range update code was updated to work as
expected when using a separate opacity array/component.