[utils] Vertical slice extraction

A small command line utility for constructing a 2d vertical slice out of 2d horizontal layers. This is basically the setup we need for AREHS right now.

Note, that the tool uses an unfortunate workaround, where I save the constructed geometry to a gml file, remove the original geometry and load the previously saved geometry again right away. The reason is that GMSH was not able to handle the original geometry. @TomFischer has hypothised that this is due to a bug during merging geometries. We noticed a similar behaviour with a different project previously but were not able to find any specifics so far. In any case, I've encapsulated the workaround in a separate function (consolidateGeometry()) so it should be very easy to remove whenever the geometry issue gets fixed.

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