Optimizing complex balance

[vyudu]

Some performance/scalability improvements for the complex balance check:

1. Implementing a more efficient way to generate the spanning trees in a graph
2. Computing the tree weights one-by-one to reduce the space overhead
3. Is there a more efficient way to check that $D^T \text{ Ln } \rho$ is in the image of $S$?

[vyudu]

Also, need to add a test checking conditional complex balance, i.e. a reaction network that is complex balance for some set of rate parameters but not for a different set.