Formalize nested solver implementation

[ghackebeil]

Things to address:

• Setting up the communicator tree and listeners could be made easier
• Allow load balance and timing statistics to be sent up to the root solver

[ghackebeil]

I've added a more formalized implementation of a two layer scheme in #7. For this, there is no need to build a tree of communicators. No plans to add support for deeper nesting until there is a motivating example.