Added nearest_point_tetrahedron_3 function to allow for
closest point queries for tet meshes.
Added 3d example to distancequeries demo. Streamlined demo file.
Made distancequeries demo header comply to standard DOLFIN header.
Remove partial specialization of Distance struct for Tetrahedron class
which threw an exception.
Small fix in in nearest_point_tetrahedron implementation
Added tests for distance queries in 2d and 3d
Merged in massing/dolfin/closest-point-3d (pull request #10)

Implemented distance queries for 3d meshes