Port vertex welding code to GPUs via RAJA

[kennyweiss]

The vertex welding code converts a "soup of triangles", e.g. from an STL mesh, into a mesh whose triangles share vertices.

The code currently depends on std library functionality, e.g. hashmaps (std::unordered_map) and std::hash,
as well as spin::RectangularLattice (which is being ported in #677). GPU-ready hashmaps are the subject of #630

axom/src/axom/quest/MeshTester.hpp

Lines 367 to 400 in 18f7408

	/*!
	* \brief Mesh repair function to weld vertices that are closer than \a eps
	*
	* \param [in,out] surface_mesh A pointer to a pointer to a triangle mesh
	* \param [in] eps Distance threshold for welding vertices (using the max norm)
	*
	* \pre \a eps must be greater than zero
	* \pre \a surface_mesh is a pointer to a pointer to a non-null triangle mesh.
	* \post The triangles of \a surface_mesh are reindexed using the welded
	* vertices and degenerate triangles are removed. The mesh can still contain
	* vertices that are not referenced by any triangles.
	*
	* This utility function repairs an input triangle mesh (embedded in three
	* dimensional space) by 'welding' vertices that are closer than \a eps.
	* The vertices are quantized to an integer lattice with spacing \a eps
	* and vertices that fall into the same cell on this lattice are identified.
	* All identified vertices are given the coordinates of the first such vertex
	* and all incident triangles use the same index for this vertex.
	*
	* The input mesh can be a "soup of triangles", where the vertices
	* of adjacent triangles have distinct indices. After running this function,
	* vertices that are closer than \a eps are welded, and their incident
	* triangles use the new vertex indices. Thus, the output is an
	* "indexed triangle mesh".
	*
	* This function also removes degenerate triangles from the mesh. These
	* are triangles without three distinct vertices after the welding.
	*
	* \note This function is destructive. It modifies the input triangle
	* mesh in place.
	* \note The distance metric in this function uses the "max" norm (l_inf).
	*/
	void weldTriMeshVertices(mint::UnstructuredMesh<mint::SINGLE_SHAPE>** surface_mesh,
	double eps);

This might be helpful for improved robustness in the SignedDistance query. See, e.g. #703.

[rhornung67]

Assign after GPU hashmap is available.