Implement StochOptFormat

[odow]

This PR implements a reader and writer for StochOptFormat.

It is still very much experimental and a work-in-progress, but it works!

TODOs

• Documentation
• Error messages
  • Throw if existing cuts
  • Throw if belief states
  • Throw if objective states
• Handle changes in the objective coefficients
• Detect set_normalized_rhs
• Detect set_normalized_coefficient
• Detect bound changes
• What to do for test_scenarios?
• Support maximization problems
• Detect subproblems with mixed senses
• Provide a way to give an initial bound

cc @joaquimg

[odow]

Merging since this works well enough for a first-pass and is getting quite big to review. I'll open issues for the remaining TODOs.