Un-deprecate extract boundary in ExtractStructured filter

This method was deprecated because it was not well explained in the documentation nor was it used in VTK-m or Ascent. However, it is used in VTK, and VTK has a bit more documentation on what this feature is supposed to do. Thus this feature has been un-deprecated so that it will continue to stick around. Also, additional documentation is provided to describe the feature, and the testing has been expanded a bit.

Backport: release