Complex balancing

[vyudu]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

A first pass at the complex-balance implementation. Does not seem like Graphs.jl has a matrix-tree implementation, so I have the matrix tree implementation here for now, but I'll work on adding it as a PR to Graphs.jl (and in particular implementing a method of generating the spanning trees that doesn't simply generate every combination of n-1 edges).

[isaacsas]

Let me start by saying this is a great first PR!

Here are a first set of comments -- these are more general Julia practices / Catalyst internals comments than method specifc or performance specific.

I need to finish going through the paper and then I will give a second review more focused on the algorithmic details, but you can at least address these for now.

[isaacsas]

@vyudu can you merge master into this? Do you know how to do that? (If not I can explain to you on Slack.)

I usually use Gitkraken to do that as I find it easier than merging via the commandline.

[TorkelE]

I second that GitKraken is a really nice tool, if you haven't checked it out yet.

[isaacsas]

Can you add one more test that uses a network that is complex balanced with multiple nonlinearities and non-trivial rate expressions (i.e. reactions like k1*k2, 3A + 2B --> 3*C)? Just to stress the code a bit more.

In terms of performance optimization I think that can be a followup. So I'm happy to merge once you address this last set of comments.