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Correct handling of gravitational flux contributions requires the
mpfa framework to be completely re-designed.

The Doxygen comment `//!` refers to the line after the comment. If
the comment should refer to the same line, `//!<` has to be used.

This implies that all processes use the same interaction volume on the boundaries. We typically
use the o-method on boundaries, where this is guaranteed.

TODO: implement ficks and fouriers law, as well as the interaction volume data handles
accordingly for problems involving diffusion/heat conduction