Copy Threshold output to a CellSetExplicit

Perhaps a better title for this change would be "Make the Threshold filter not totally useless."

A long standing issue with the Threshold filter is that its output CellSet was stored in a CellSetPermutation. This made Threshold hyper- efficient because it required hardly any data movement to implement. However, the problem was that any other unit that had to use the CellSet failed. To have VTK-m handle that output correctly in other filters and writers, they all would have to check for the existance of CellSetPermutation. And CellSetPermutation is templated on the CellSet type it is permuting, so all units would have to compile special cases for all these combinations. This is not likely to be feasible in any static solution.

The simple solution, implemented here, is to deep copy the cells to a CellSetExplicit, which is a known type that is already used everywhere in VTK-m. The solution is a bit disappointing since it requires more memory and time to build. But it is on par with solutions in other libraries (like VTK). And it really does not matter how efficient the old solution was if it was useless.