Add PPP2 algorithm for computing contour trees

This merge request adds:

• a new implementation of the contour tree algorithm (Parallel Peak Pruning 2) which enables the efficient computation of fully augmented contour trees and computation of measures/statistics over the full tree
• computation of the contour tree that is consistent with marching cubes isosurface extraction
• an example application for computing the contour tree using the new PPP2 algorithm
• an example application for selection of isovalues bases on the fully augmented contour tree compute using PPP2

This merge request furthermore extends the current implementation of the contour tree (PPP1) to enable

• the computation of the fully augmented contour tree. Previously, only the contour tree (without augmentation) was supported. While the augmentation using PPP1 is not efficient, it serves as a reference for comparison with the new PPP2 algorithm.
• the computation of the contour tree that is consistent with the interpolant assumed by the marching cubes algorithm

With the new implementation of the contour tree we can now compute the fully augmented contour tree (i.e., the contour tree including all regular vertices), which is critical to compute statistics and segmentations based on the contour tree.

This code was created as part of the ECP Alpine project. @ghweber @berkgeveci