Linear Tree Implementation

[bammari]

This PR now allows users to embed linear model decision trees (trained with the linear-tree package https://github.com/cerlymarco/linear-tree) within OMLT. We include several univariate and bivariate tests to ensure the several implemented formulations are functional. Furthermore, this PR includes a preliminary Jupyter notebook that shows how to embed these linear model decision trees. using OMLT

In addition to this implementation, this PR also rewrites the way scaled_input_bounds are defined within the _setup_scaled_inputs_and_outputs function in formulation.py. Specifically, we pass a dictionary of tuples containing lower and upper bounds rather than constructing a bounds rule. test_maxpool_FullSpaceNNFormulation was failing without this change.

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[rmisener]

Just noticed something small while working through some material, this should be easy.

[rmisener]

Would it be possible to arrange the placement of the notebooks so that bo_with_trees.ipynb is in the same folder as this new notebook? At least it's confusing to me why the notebooks for the trees are in separate places. If linear trees are so different from the GBT implementations that they do need to be in a different folder, perhaps add one more folder so that it's notebooks/trees and then have everything inside?

[bammari]

Per your suggestion, I consolidated both of these notebooks in notebooks/trees.

In addition, I updated docs/notebooks.rst.

[rmisener]

Please also add a description of the new Jupyter notebook in docs/notebooks.rst

[rmisener]

Going through minor things and want to make sure @bammari gets credit for this work. Would you please also add to this PR:

In README.rst

• Please add a reference to the Computers & Chemical Engineering paper. State that if anyone uses the linear tree implementation, they should cite the C&CE paper.
• Please add @bammari to the list of contributors at the bottom and state his funding.

In the new notebook, please add the C&CE paper as a reference. As an example of how to do this, see bo_with_trees.ipynb

[bammari]

Going through minor things and want to make sure @bammari gets credit for this work. Would you please also add to this PR:

In README.rst

• Please add a reference to the Computers & Chemical Engineering paper. State that if anyone uses the linear tree implementation, they should cite the C&CE paper.
• Please add @bammari to the list of contributors at the bottom and state his funding.

In the new notebook, please add the C&CE paper as a reference. As an example of how to do this, see bo_with_trees.ipynb

Added reference to CACE paper in both the notebook and Readme.rst. I also updated the OMLT paper reference from Arxiv to Journal of ML Research in README.rst. In addition, added contribution statement - Bashar