Knepley/feature dm remove hybrid

This is a huge rewrite of Plex to allow any number of cell types in a single mesh. Interpolation, refinement, and submeshes for prism cells should now work smoothly. We allow both oriented and tensor-product prisms in the same mesh. Moreover, cell refiners have been abstracted. We currently have two (regular refinement, and conversion to hexes). The plan is to add more, such as local extrusion and conversion to simplices.