[MeL] Implement NodeAdjacencyTable.
[MeL] Add setMatrixSparsity.
[PL] GW: Use node adjacency table.
[MeL] Document Node::setConnectedNodes().
[MeL] Rewrite and enable setNodesConnectedByElements().

The rewrite gives factor 3 speedup.
[MeL] Use nodes' connectivity information.

Adjacency information is already computed for
each node; transform to ids in createTable().
[MeL] Remove unnecessary adjacency degree hint.