list of sets of unconnected subgraphs that can be merged

[brendan8229]

In #59 we found connected subgraphs that can be merged.

If there is a parallel partition with graphs G1 and G2 and both of these are sequential, then we can also merge subsequences for G1 with subsequences of G2.

This certainly expands the list of possible merges we consider. It's possible that this is then all the subsets that can be merged, but we have to look more closely at that.

Write a method that builds on #59 and looks in #59 for subgraphs whose nodes can be merged and write these to a list. Make this list separate from the list created in #59