Cleanup / reimplement HangingNodesConstraints

We are currently cleaning up the pdelab code; the hanging nodes assembler is in a very sad state and doesn't use all the new shiny features. On the other hand, I doubt that it is worth the effort updating a partially working hangning-nodes-assembler.

Required cleanup:

• avoid use of EntityPointer
• create copies of Entity and Intersection

Some notes on the general problems:

• it doesn't work properly for composite spaces
• it doesn't work for higher order spaces
• it doesn't work for mixed-type grids