Added N^2 node search option back into SerialMesh::stitch_meshes.

This option is more robust in the case that we have tolerance issues that lead to less accurate alignment of the nodes that are to be stitched.

The N^2 search was the initial implementation, but was replaced by @jwpeterson due to its slowness. But it now avoids the expensive loop over other_mesh (using the reverse node map introduced by @jwpeterson), so it's not so slow anymore (I haven't done detailed comparisons yet though).