File Based Logging

[wgledbetter]

We need better introspection capabilities into the operations of the optimizer. This will help users understand why their problem may be poorly formed and identify errors in their dynamics, constraints, and objectives.

I've had good experiences with spdlog, and I can copy over some utilities I've written for other projects to help get started.

[jbpezent]

@wgledbetter Per our discussion last Friday, I'll leave the implementation of the file-based CPP side logging up to you. I'll handle the separate issue of letting users retrieve the violations of individual constraints through the phase/ocp interfaces.