Can set an absurd value in only the x direction

This is for fixing the case that LIBMESH_DIM < 3
We can't add z-component of BC if LIBMESH_DIM < 3
We can't grab the (2,2) element if LIBMESH_DIM < 3
Only set z-component of gradient if LIBMESH_DIM > 2
New autoconf test to determine LIBMESH_DIM

This is useful because we can then SKIP tests that require
a certain LIBMESH_DIM and that LIBMESH_DIM is not present
in the configured LIBMESH_DIR.
Test for LIBMESH_DIM
Skip these tests if LIBMESH_DIM != 3
Merge pull request #123 from pbauman/2donly_fixes

2D only fixes