Make triangulate implementation more general

The triangulate and tetrahedra worklet implementations were for some
reason grabbing raw arrays out of `CellSetExplicit` and using those for
lookup tables. Rather than do that, simply use a
`WorkletVisitCellsWithPoints` automatically provide the values of these
cells. The advantage of this approach is that it simplifies the code and
also generalize it to cell sets that do not provide this explicit
representation. For example, the new implementation also works with
`CellSetExtrude`.
5 jobs for triangulate-single-cell-type
Status Name Job ID Coverage
  External
passed cdash-buildbot

passed vtk-m-dragnipur-osx-shared-debug+32bit_ids+64bit_coords+clang+tbb

00:01:37

passed vtk-m-dragnipur-osx-static-release+64bit_coords+clang+tbb

00:01:53

pending vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb
pending vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb