Instead convert to/from bytes in UndirectedGraph.

Store graphs and coefficients in a SQLite database.

We don't need this as we don't use sparse graphs.

Also make the ordering of those vectors deterministic. This comes at a
slight cost, at least in memory usage.

Return either vertex orbits or edge orbits, not both.

Use this option for the graphs produced in expanding_differential, so
that in the next step, only one representative of each edge orbit can be
contracted. So far we do not use this option for the graphs produced by
contractions. (It could be used in the future to produce coboundaries.)