Add true deprecation warning to pyomo command

[mrmundt]

Fixes #160

Summary/Motivation:

This PR adds a true deprecation warning to the pyomo command.

Changes proposed in this PR:

• Print deprecation warning if pyomo is called without a subcommand

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.

[Robbybp]

This just changes how a warning is displayed, and even has a test

[codecov]

Codecov Report

Base: 82.23% // Head: 87.01% // Increases project coverage by +4.77% 🎉

Coverage data is based on head (7ccced6) compared to base (6ef8840).
Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2740      +/-   ##
==========================================
+ Coverage   82.23%   87.01%   +4.77%     
==========================================
  Files         771      771              
  Lines       86306    86307       +1     
==========================================
+ Hits        70975    75101    +4126     
+ Misses      15331    11206    -4125     

Flag	Coverage Δ
linux	84.18% <66.66%> (?)
osx	73.65% <66.66%> (?)
other	84.36% <66.66%> (?)
win	81.55% <66.66%> (?)

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

Impacted Files	Coverage Δ
pyomo/scripting/pyomo_main.py	72.34% <66.66%> (+7.12%)	⬆️
pyomo/util/subsystems.py	89.13% <0.00%> (-8.70%)	⬇️
...ontrib/appsi/utils/collect_vars_and_named_exprs.py	93.75% <0.00%> (-6.25%)	⬇️
pyomo/core/base/external.py	95.43% <0.00%> (-3.16%)	⬇️
pyomo/contrib/appsi/writers/nl_writer.py	84.39% <0.00%> (-2.32%)	⬇️
pyomo/core/expr/compare.py	96.52% <0.00%> (-1.74%)	⬇️
pyomo/repn/plugins/ampl/ampl_.py	81.40% <0.00%> (-1.47%)	⬇️
pyomo/contrib/appsi/base.py	84.94% <0.00%> (-0.26%)	⬇️
pyomo/repn/plugins/nl_writer.py	93.60% <0.00%> (-0.16%)	⬇️
pyomo/contrib/appsi/solvers/ipopt.py	88.51% <0.00%> (ø)
... and 108 more

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