Fix pattern semantic equivalence comparison expression DNF transformer bug

[chisholm]

Fixes #533 .

This PR fixes the comparison expression DNF transformer to catch the exception caused by inconsistent SCO types among comparison expression AND operands, and drop the AND expression. This resolves the problem and additionally enables a nice simplification. For example, the example from the issue was:

[(a:b=1 OR b:c=1) AND (b:d=1 OR c:d=1)]

which would have transformed to:

[(a:b=1 AND b:d=1) OR (a:b=1 AND c:d=1) OR (b:c=1 AND b:d=1) OR (b:c=1 AND c:d=1)]

The transformer will now drop the first, second, and fourth AND expressions due to their mixed SCO types (which means they'd be impossible to satisfy anyway). The result is simply:

[b:c=1 AND b:d=1]

which is semantically equivalent to the original: only a b SCO could have satisfied it, which is obvious in its simplified form.

Also:

• Added a couple more unit tests to test this situation. All of the original tests were designed to create different AST structures to exercise the transformers, and were not thought of in terms of mixing SCO types. So none of the tests caught the error.
• Changed _dupe_ast() in the comparison expression transformer module to dupe simple \ \ \ comparison expressions. Previously it did not do this because I think at the time, no transformation of that type of simple comparison expression happened. So there was no need. But special context-sensitive constant canonicalization was added, which does change that type of comparison expression (in particular, the constant portion). So at the very least, the old comment about those expressions not changing was wrong. The stated rationale was wrong. I decided since the rationale was wrong now, I should probably go ahead and dupe them.
• The patterns.py change allowed the _BooleanExpression constructor to support an iterator for its operands parameter. The old code implied the parameter value must support iteration over the values twice, which is not possible with an iterator. It was a very minor change.

[codecov-commenter]

Codecov Report

Merging #534 (3086b7a) into master (81550ca) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #534   +/-   ##
=======================================
  Coverage   87.28%   87.29%           
=======================================
  Files         153      153           
  Lines       18025    18033    +8     
=======================================
+ Hits        15734    15742    +8     
  Misses       2291     2291           

Impacted Files	Coverage Δ
stix2/test/test_pattern_equivalence.py	100.00% <ø> (ø)
stix2/equivalence/pattern/transform/comparison.py	74.62% <100.00%> (+1.61%)	:arrow_up:
stix2/patterns.py	62.01% <100.00%> (ø)

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