Fix halfface _plane

[romanarust]

Suggestion for changing the internal _plane storage of the Halfface from _plane[u][v][w] to _plane[u][v][fkey].

Reason:
Using u, v, w indices is problematic when 2 adjacent faces (f1key, f2key) share 3 vertices u, v, w.
if they also belong to the same cell, then.
_plane[u][v][w] = ckey
and
_plane[w][v][u] = ckey
however, _plane[w][v][u] is supposed to be the opposite cell to _plane[u][v][w].

Using the face key instead, the assignment becomes unique.
_plane[u][v][f1key] = ckey
_plane[v][u][f2key] = ckey
opposite cells:
_plane[v][u][f1key] = ckey_opposite
_plane[u][v][f2key] = ckey_opposite

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

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• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
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• I have added tests that prove my fix is effective or that my feature works.
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