Implement Interval PointFrom operator

[dehall]

This PR implements the point from interval operator. (closes #163)

define PointFromUnit2Interval: point from Interval[2,2]

If the given interval is a "unit interval", aka it only contains a single point, this operator returns that point. Otherwise, it throws a runtime error.

To get that single point, it just checks if start() is equal to end(). If both bounds of the interval are null, it returns null, hence checking with both == and equals.

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• Code diff has been done and been reviewed (it does not contain: additional white space, not applicable code changes, debug statements, etc.)
• Tests are included and test edge cases
• Tests have been run locally and pass
• Code coverage has not gone down and all code touched or added is covered.
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• All dependent libraries are appropriately updated or have a corresponding PR related to this change
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• Code is maintainable and reusable, reuses existing code and infrastructure where appropriate, and accomplishes the task’s purpose
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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.43%. Comparing base (5c28fd1) to head (1f32af0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   88.41%   88.43%   +0.02%     
==========================================
  Files          54       54              
  Lines        4832     4843      +11     
  Branches     1359     1361       +2     
==========================================
+ Hits         4272     4283      +11     
  Misses        368      368              
  Partials      192      192              

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