[Engine] Rename VolumeFromTetrahedrons to VolumeFromVolumetricElements

[alxbilger]

Because it also computes volume from hexas.

The same should be done for VolumeFromTriangles in another PR.

[with-all-tests]

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[fredroy]

While you are at renaming, I would suggest to rename this example like VolumeFromVolumetricElements_Tetrahedra. scn or something like that, to keep the (unwritten?) convention: name of scene = name of the component