Improve box geometry helper implementation

What this MR does / why does DuMux need it:

Makes BoxGeometryHelper a thin wrapper around an element geometry. All points are constructed directly from the reference element via transformation given the element geometry. This means the helper can be cheaply constructed and used to create scv and scvf geometries on-the-fly.

This should serve as a blueprint for other control volume schemes. Exploiting this change, the memory footprint of scv and scvfs can be much improved (#1173) subsequently.

• Better interface for boundary scvfs (not depending on intersection object)

Notes for the reviewer

I couldn't measure any performance penalty, although some points are transformed more often than before.

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