LinearExpression should be immutable like all other expression nodes

[michaelbynum]

Summary/Motivation:

As far as I know, LinearExpression is the only expression object in the expression system that is mutable. This PR updates LinearExpression so that it is immutable.

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[codecov[bot]]

Codecov Report

Merging #2330 (f1c1a0a) into main (b48c886) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2330      +/-   ##
==========================================
- Coverage   85.61%   85.61%   -0.01%     
==========================================
  Files         618      618              
  Lines       76206    76237      +31     
==========================================
+ Hits        65241    65267      +26     
- Misses      10965    10970       +5     

Flag	Coverage Δ
linux	82.43% <100.00%> (+<0.01%)	:arrow_up:
osx	72.68% <100.00%> (+0.01%)	:arrow_up:
other	82.38% <100.00%> (+<0.01%)	:arrow_up:
win	79.47% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/core/expr/numeric_expr.py	98.34% <100.00%> (+0.05%)	:arrow_up:
pyomo/common/env.py	89.15% <0.00%> (-2.01%)	:arrow_down:

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[blnicho]

This change needs some discussion before merging. At a minimum I think some documentation updates are needed in https://github.com/Pyomo/pyomo/blame/main/doc/OnlineDocs/advanced_topics/linearexpression.rst and maybe https://github.com/Pyomo/pyomo/blob/main/doc/OnlineDocs/developer_reference/expressions/design.rst

[michaelbynum]

@blnicho, I completely agree. I want to make sure everyone agrees with these changes before I spend time on the documentation, though.

[mrmundt]

Addressed in another branch by @jsiirola.