Benders: rename master problem to root problem

[michaelbynum]

Summary/Motivation:

This renames the master problem to root in the contrib benders package

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

Merging #1828 (ccb84f2) into master (1785b31) will increase coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1828      +/-   ##
==========================================
+ Coverage   78.62%   78.87%   +0.25%     
==========================================
  Files         554      554              
  Lines       68814    70642    +1828     
==========================================
+ Hits        54104    55720    +1616     
- Misses      14710    14922     +212     

Impacted Files	Coverage Δ
pyomo/contrib/benders/benders_cuts.py	88.53% <100.00%> (ø)
pyomo/common/fileutils.py	91.37% <0.00%> (-4.21%)	⬇️
pyomo/core/base/piecewise.py	84.64% <0.00%> (-3.03%)	⬇️
pyomo/contrib/mindtpy/iterate.py	78.26% <0.00%> (-2.49%)	⬇️
pyomo/solvers/plugins/solvers/CPLEX.py	82.89% <0.00%> (-2.45%)	⬇️
pyomo/core/preprocess/simple_preprocessor.py	20.58% <0.00%> (-0.85%)	⬇️
pyomo/dataportal/parse_datacmds.py	90.44% <0.00%> (-0.30%)	⬇️
pyomo/common/unittest.py	100.00% <0.00%> (ø)
pyomo/core/base/reference.py	97.62% <0.00%> (+0.02%)	⬆️
pyomo/opt/base/solvers.py	86.63% <0.00%> (+0.02%)	⬆️
... and 13 more

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