Fix BIHTree::find_bounding_box

- add projection_le, projection_ge methods to BoundingBox
  ... for comparison with smae safe factor as in intersect method
- use these in find_bounding_box to deal correctly with lower dim elemnts
- modified bih_tree_test (WIP)
- bih_node completely inlined
Fix noncompatible P0 assembly.

- fix noncompatible P0 assembly.
- add id() method to Element class
Minor cleanup in mesh.

- remove some old commented code
- remove or comment out some debug prints
- add missing exc_common.hh file
- remove unecessary member sigma from Neighbour class
Resurrect P0, P1 coupling and error calculation.

- make P1 working
- remove some old code
- make error calculation work
- make error calculation switch in input file