WIP: Add SymbolicRegressionModel that uses big-m approach

[radhakrishnatg]

Summary/Motivation:

This PR adds refactored code for constructing the model using the Big-m approach for transforming disjuncts. Most of the constraints have been added, except redundancy cuts. Need to work on that.

To run this code, we need the changes in https://github.com/radhakrishnatg/pyomo/tree/custom-block
So, for now, instead of pip installing Pyomo: clone the repo -> install Pyomo by running python setup.py install in the environment.

Changes proposed in this PR:

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[radhakrishnatg]

@dfardis Please do not merge this PR yet. I want you to review the entire code, test it out on a few examples, and merge it after you make sure everything is in place.
For now, please ignore hul_operators.py and utils.py files.