Implement thermosyphon

[jcallaham]

Implementing the chaotic thermosyphon from JC's paper with rotary control of the inner cylinder. Objectives could be either stabilization or (probably more interesting) maximizing heat transfer.

• Create mesh
• Add new BoussinesqFlowConfig for problems with heat transfer
• Solve steady conduction problem with Thermosyphon flow
• Add semi-implicit scalar solve to the timestepper for heat transfer
• Validate results against JC's paper
• Add and test control