Update benchmark for rename UnitCube --> UnitCubeMesh
Add benchmark for interpolation between meshes
Add mutable shared bounding box tree member to Mesh class.
Benchmark inter-mesh interpolation between DOLFIN and CGAL:

CGAL:   0.944838047028 s
DOLFIN: 0.713277101517 s

Both options still implemented in the Function class and selected
via parameter use_cgal_intersection. New implementation looks correct
and is faster than CGAL so the next step will be to remove the CGAL
implementation in the Function class.
Remove use of CGAL from Function class. Cleanups needed.
Remove old mesh intersection demos. New demos relevant for the new
functionality will be readded later when more functionality is in place.
Remove intersection functionality from Mesh class. Only remaining
usage was in PointSource class.
Clean out CGAL intersection library. Andre's excellent work on the
CGAL intersection library will always have a place in our memories
(and in the git space-time continuum)!
Remove CGAL versions of computational geometry benchmarks to make the
`next` buildbot green again, and adjust parameters in benchmarks to
make them "future-proof" (larger).

Current local timings for the record:

bench_geometry_bounding_box_tree_build_cpp                      7.37162 s
bench_geometry_bounding_box_tree_compute_closest_entity_cpp     7.05838 s
bench_geometry_bounding_box_tree_compute_entity_collisions_cpp  5.57038 s
Soem clean up of null space demos.
Remove transpose null space function from linear solver - not used.
Update Trilinos ML pc wrapper for new VectorSpaceBasis.
Add missing include.
Merged in logg/topic-remove-cgal-intersection-library (pull request #44)

Remove CGAL intersection library
Remove unused typedef.
Merge branch 'garth/cleanup-singular-demo'
Merge branch 'garth/ml-nullspace-update'
Merge branch 'garth/ml-nullspace-update' into garth/cleanup-singular-demo
Fix for nullspace setting picked up by clang.
Merge branch 'garth/cleanup-singular-demo'