First interpolates from the cells to the vertices and then from the face
vertices to the face centres.  This may be a useful scheme for calculating
gradients on bad/tet meshes when used in conjunction with the Gauss gradient
scheme.

Removed test objs.

Weird one. Reported crash with gcc452 with end() iterator returning a copy.
The end() iterator returned was not initialised. No idea why & could not
repeat.