Fix pyramid face corner

[lukasdreyer]

Describe your changes here:
Should adress #862. Need discussion with @Davknapp regarding current standards for vertex and face enumeration of pyramids.

This fixes t8_dpyramid_face_corner to fit to t8_dpyramid_face_neighbour and t8_dpyramid_vertex_reference_coords.
Attention: The relation of faces to vertices differs for the two pyramid types!

In the future, this relation should be the same for every element shape.

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[lukasdreyer]

The newly added test fails in parallel because of offsets, but it passes on this branch in serial, and fails in serial on main.

[lukasdreyer]

Merging main and making two errors in the _ext function that I introduced let me think about whether we really need the _ext version when switching to t8code 2.0, or if we can replace the standard version of that function