Fix hybrid bigm formulation for linear trees

[emma58]

The changes in #163 included changes to the hybrid bigm formulation for linear tree that, while mathematically equivalent, made for a larger formulation in terms of number of constraints. This PR corrects that: It still uses the gdp.bound_pretransformation to generate the constraints bounding the features values for each leaf, but it manually transforms the constraints setting the output value to the leaf's linear function, equivalently to @bammari's original implementation. In addition it adds a test to check that the size of the resulting formulation is what is expected.

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[jalving]

It looks like the graph neural network notebook is failing. i'll have a look soon.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.31%. Comparing base (321a2e2) to head (cf87696).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #164      +/-   ##
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+ Coverage   90.27%   90.31%   +0.04%     
==========================================
  Files          32       32              
  Lines        2005     2014       +9     
  Branches      460      316     -144     
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+ Hits         1810     1819       +9     
  Misses        123      123              
  Partials       72       72              

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[jalving]

sorry this took awhile to get to @emma58