fix: Polycone::addZPlanes when already 2 planes defined

[wdconinc]

When a valid Polycone exists with 2 (or more) z-planes, calling addZPlanes with a single z-plane is well-defined.

BEGINRELEASENOTES

• fix adding individual planes to existing Polycone

ENDRELEASENOTES

[github-actions[bot]]

Test Results

    8 files      8 suites   2h 33m 27s :stopwatch:   363 tests   362 :white_check_mark: 0 :zzz: 1 :x: 1 428 runs  1 427 :white_check_mark: 0 :zzz: 1 :x:

For more details on these failures, see this check.

Results for commit 641358c8.

:recycle: This comment has been updated with latest results.