Observer gurobi refactor

[michaelbynum]

Summary/Motivation:

This PR refactors the gurobi interfaces to achieve two goals:

• reduce duplicate code
• use the new contrib.observer package

Changes proposed in this PR:

• create a directory for gurobi now that there are 4 files
• create a base class for all of the interfaces that use gurobipy, making the direct and persistent interfaces more consistent (e.g., tc_map, postsolve, solve, mipstart)
• modify the persistent interface to use the contrib.observer package
• add mipstart functionality to gurobi direct and gurobi minlp
• add a minimum gurobipy version to the direct interface (addMConstr was not added until version 9)
• simplified some of the persistent interface

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

❌ Patch coverage is 91.10915% with 101 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.25%. Comparing base (4696230) to head (93bb118).

Files with missing lines	Patch %	Lines
...contrib/solver/solvers/gurobi/gurobi_persistent.py	89.24%	83 Missing ⚠️
...ontrib/solver/solvers/gurobi/gurobi_direct_base.py	94.00%	16 Missing ⚠️
...omo/contrib/solver/solvers/gurobi/gurobi_direct.py	97.05%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3698   +/-   ##
=======================================
  Coverage   89.24%   89.25%           
=======================================
  Files         909      911    +2     
  Lines      105536   105551   +15     
=======================================
+ Hits        94190    94212   +22     
+ Misses      11346    11339    -7     

Flag	Coverage Δ
builders	29.15% <24.29%> (+0.05%)	⬆️
default	85.82% <90.66%> (?)
expensive	35.64% <24.29%> (?)
linux	86.70% <91.02%> (-2.30%)	⬇️
linux_other	86.70% <91.02%> (+<0.01%)	⬆️
osx	82.88% <91.02%> (+<0.01%)	⬆️
win	84.44% <91.02%> (+0.01%)	⬆️

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