Add option to skip fixing inputs in sequential decomposition

[alma-walmsley]

Summary/Motivation:

I am working on a problem (within IDAES) where pyomo network's sequential decomposition is used to initialize the model. For tear ports, I want to either:

• Fix the variables on that port directly, or
• Remove a degree of freedom by adding a constraint for an expression at that port.

For example, if the port has the variables [flow_mol, pressure, enth_mol], and temperature is an expression of pressure and enth_mol, then I want the ability to unfix enth_mol and constrain temperature to some value to determine the enthalpy.

In pyomo network's case, when running sequential decomposition, all the variables at the port get fixed if they are currently unfixed. I assume this is to prevent user error from too many degrees of freedom.

However, in my case, my model gets over-defined, since a constraint is added to remove a degree of freedom when a variable is unfixed, but those variables get fixed anyway when sequential decomposition is run.

Changes proposed in this PR:

• Add an option in sequential decomposition to skip fixing all the variables at the tear ports. Defaults to False. Variables can be fixed manually or through the set_guesses_for method.

Legal Acknowledgement

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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

All modified and coverable lines are covered by tests ✅

Project coverage is 88.71%. Comparing base (eb8a6b5) to head (6b1ebce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3442      +/-   ##
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- Coverage   88.71%   88.71%   -0.01%     
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  Files         882      882              
  Lines      100622   100625       +3     
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+ Hits        89271    89272       +1     
- Misses      11351    11353       +2     

Flag	Coverage Δ
linux	86.18% <100.00%> (-0.01%)	⬇️
osx	76.20% <100.00%> (+<0.01%)	⬆️
other	86.67% <100.00%> (-0.01%)	⬇️
win	84.64% <100.00%> (+<0.01%)	⬆️

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