Feature/refactor and cleanup

[gparadis]

• Refactored code to move some helper functions (that we added as part of the new parallel optimization problem building update) into a new forest_helper module.
• Fixed a few small (but show-stopper) bugs in the parallel model building code path (fixed now, all good), and re-ran all the examples to make sure that they still work.
• Added some new bits to some of the examples to reveal the new parallel model building functions, updated the examples to reflect the new HiGHS LP solver default backend, and expanded some of the example notebook comments.
• Rewrote all example code (and code in examples/util.py helper module) to systematically use double quotes instead of single quotes (for consistency and improved readability), and refactored some of the code to improve code style consistency, and added docstrings to all functions in the examples/util.py helper module.