Piecewise Linear transformations: Logarithmic and nested inner GDP representation

[sadavis1]

Fixes # .

Summary/Motivation:

We want to add piecewise-linear to GDP and to MIP transformations from Vielma 2010 so they can be used in Bashar's PWL representations paper.

Changes proposed in this PR:

• Add disaggregated_logarithmic transformation for PW linear, direct to mip as in Vielma
• Add "nested_inner_repn" transformation, which should be a path to GDP-ification of Vielma's log transformations by identifying some variables, once some changes are made to gdp transformations to support it. Also, this transformation is somewhat interesting in its own right, because it is often faster than the inner representation despite not actually having the logarithmic variable count.
• Rename class PiecewiseLinearToGDP to PiecewiseLinearTransformationBase, since it isn't GDP-specific and I'm using it for a MIP.

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1. I agree my contributions are submitted under the BSD license.
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[sadavis1]

Fixed typos and some other minor issues.

[sadavis1]

This should be ready now. The linux/3.12 test failure seems to be spurious.

[sadavis1]

Oh... oops. I seem to have done something strange with git. One minute..

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.05263% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 86.55%. Comparing base (69082ac) to head (e954043).

Files	Patch %	Lines
...o/contrib/piecewise/transform/nested_inner_repn.py	93.67%	5 Missing :warning:
...b/piecewise/transform/disaggregated_logarithmic.py	98.36%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3036      +/-   ##
==========================================
- Coverage   88.39%   86.55%   -1.85%     
==========================================
  Files         847      849       +2     
  Lines       95263    95405     +142     
==========================================
- Hits        84212    82582    -1630     
- Misses      11051    12823    +1772     

Flag	Coverage Δ
linux	?
osx	?
other	83.83% <96.05%> (-2.70%)	:arrow_down:
win	83.83% <96.05%> (+<0.01%)	:arrow_up:

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