Add 3D Kdtree and Nearest Neighbor Search

caseywang777requested to merge
caseywang777/vtk-m:kdtree into master

Add 3D Kdtree and Nearest Neighbor Search. This is going to increase the speed of particle search in the space. Tree construction reference: "Portable Data-Parallel Visualization and Analysis in Distributed Memory Environments" (LDAV 2013).

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