#196 Add node ordering algorithms, including dependencies, for later use by DCSH

Now that we have graphs (!332 (merged)), here are the algorithms that work on graphs:

• Rooted level structure computation.
• Pseudo-peripheral node finders.
• Node orderers.
• Variable orderers, based on the node orderers.

Note that:

• I tried to stay as close as possible to the original papers. We thus have two pseudo-peripheral node finders, one for each of the node ordering algorithms, such that they use what they describe in their papers.
• Weighted Cuthill-McKee has been improved wrt the original paper. It now works also for multiple unconnected partitions with >1 node.
• These algorithms form the ingredients for the DCSH algorithm itself, to be contributed in a follow-up merge request.

Addresses #196 (closed)