What this MR does / why does DuMux need it:
As already done for other discretization models, e.g. MPFA or fcstaggered, the geometry helper can be deduced from the Traits class.
This offers more flexibility. Furthermore, in the
fvelementgeometry.hh the type of the helper is deduced from the gridgeometry.
Notes for the reviewer
This could help unifying grid geometries for CVFE schemes and is for example also needed to easly switch between overlapping and non-overlapping scvs by changing only the helper class
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