Optimized mesh stitching

This improves the SerialMesh::stitching_helper() function by avoiding calls to UnstructuredMesh::find_neighbors(), leading to a 66% stitching time reduction.

The former routine can be restored by passing an extra boolean flag to SerialMesh::stitching_helper().

An example is included in the miscellaneous series which demonstrates how to stitch meshes and validates the solution of a Poisson problem against the unstitched case.