Reduce computational overhead for reverse connectivity calc.
Benchmarking in VTK showed significant overhead in the computation of the reverse connectivity calculation in ConnectivityExplicitInternals::ComputeCellToPointConnectivity.
This patch adds a ReverseConnectivityBuilder that reduces the amount of time and memory needed to build the table by using an atomic histogram approach that avoids a costly radix SortByKey.
Key operations in the new helper class are templated to allow this approach to be reused by VTK-specific cell array converters.