Add BRACT for distributed contour tree computation
This merge request is Phase 1 of several to implement the distributed parallel contour tree in VTKm. This merge requests adds the base outline for the algorithm. The implementation of the details of the algorithm in the BoundaryRestrictedAugmentedContourTree.h is currently still missing. However, these will require a substantial (~3000) lines of additional code. The goal is to stage the integration process across merge requests to make the review process simpler.