Update GridTools::laplace_transform().

Update GridTools::laplace_transform().

Incomprehensively, the function looped over all faces of a cell, and then
all vertices of the face. This visited each vertex exactly 'dim' times.
This can be done easier.

While there, also fix a number of issues where we confuse 'unsigned int'
with 'types::global_dof_index'. This doesn't matter here because the
function only works in sequential settings anyway, but we should be
consistent.

Finally, instead of setting vertex locations from each adjacent cell, only
do it once by keeping track when we touch the vertex the first time.