Resolve error categorizing some Var discrete domains as "integer"

[jsiirola]

Fixes # .

Summary/Motivation:

Variables with domains defined with a domain with step 1, but not starting at an integer value were being categorized as "integer" domains. This updates is_integer() to correctly report False for discrete domains that are not rooted (start or stop) at integer values. This adds a test to exercise / verify the expected behavior.

Changes proposed in this PR:

• is_integer() should only be True if the range starts/ends at an integer
• Add tests verifying the correct behavior.

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[codecov]

Codecov Report

Patch coverage: 75.00% and no project coverage change.

Comparison is base (1cb37aa) 87.82% compared to head (039c5ec) 87.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2954   +/-   ##
=======================================
  Coverage   87.82%   87.83%           
=======================================
  Files         770      770           
  Lines       89654    89657    +3     
=======================================
+ Hits        78741    78747    +6     
+ Misses      10913    10910    -3     

Flag	Coverage Δ
linux	84.86% <75.00%> (-0.01%)	⬇️
osx	74.57% <75.00%> (-0.01%)	⬇️
other	85.04% <75.00%> (-0.01%)	⬇️
win	82.10% <75.00%> (-0.01%)	⬇️

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Files Changed	Coverage Δ
pyomo/core/base/var.py	93.59% <75.00%> (-0.18%)	⬇️

... and 1 file with indirect coverage changes

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