Replace domain comparison by type(domain) comparison

[RomeoV]

Fixes #995
Note that this bug is blocking development for #983

Fixes #995

Summary/Motivation

Variables of a cloned models don't get fixed by the Transformation (due to an ID copy issue). See the assigned issue for more information.

Changes proposed in this PR:

• change domain comparison to type(domain) comparison

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[jsiirola]

I have no issue with patching this transformation until we resolve #326, however I think I would prefer to make the "patch" part clear so that it can be removed once #326 is resolved.

[codecov-io]

Codecov Report

Merging #996 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #996      +/-   ##
==========================================
+ Coverage   69.72%   69.84%   +0.11%     
==========================================
  Files         462      462              
  Lines       72194    72494     +300     
==========================================
+ Hits        50338    50630     +292     
- Misses      21856    21864       +8

Impacted Files	Coverage Δ
pyomo/core/plugins/transform/discrete_vars.py	76.36% <100%> (ø)	⬆️
pyomo/common/errors.py	100% <0%> (ø)	⬆️
pyomo/contrib/fbbt/fbbt.py	96.12% <0%> (+0.36%)	⬆️
pyomo/contrib/gdpopt/cut_generation.py	91.33% <0%> (+1.57%)	⬆️
pyomo/contrib/gdp_bounds/compute_bounds.py	91.04% <0%> (+1.91%)	⬆️

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