Fix MeshQuality to work with CellSetSingleType

The `MeshQuality` filter only worked with `CellSetExplicit<>`. However,
`MeshQuality` should also work with `CellSetSingleType`. Fix
`MeshQuality` to work with both.

The handling of cell sets was actually worse than that. After forcing
the type to `CellSetExplicit<>`, it then applied a policy that converted
the types back to all possible cell sets. Thus, the filter made lots of
code paths that were impossible to follow.
5 jobs for cell-metrics-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

00:11:54

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

00:00:37

passed vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

00:09:43

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:34:29