UnreachableWhiteCellContradictionRule Pathfinding

[19690ao]

Description

Closes #300

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (improvement to an already existing feature)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Both the UnreachableWhiteCellContradictionRule and its corresponding CannotReachCell basic rule will now be able to use any black and/or numbered cells to determine that a cell isn't reachable from any region.

This is a screenshot of this proof file:
prtestproof.zip

If I instead try to choose every square for the rule, I'm shown exactly which ones can and can't be reached.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Chase-Grajeda]

Awesome work, passes tests and corner cases I derived