Cleanup of the use of divergence and trace

[keileg]

To map between faces and cells in a grid, we currently have the following functionality:

• fvutils.scalar_divergence, fvutils.vector_divergence are two separate methods that are not exclusive to finite volume methods. Also, the methods could be parametrized, as is done in pp.ad.GridOperators.Divergence
• There is occasional usage of a trace operator, see for instance pp.Upwind. There is however no ready functionality to get hold of a discrete trace operator (independent of dimension).

Note that this does not concern the Ad module, where the code is much cleaner (at least in a certain sense).

Suggested improvement:

1. The current functions fvutils.scalar_divergence, fvutils.vector_divergence are moved to pp.Grid and parametrized with dimension, so that one can write pp.Grid(...).divergence(dim: int=n)
2. The grid class is further equipped with methods trace(dim:int) and inv_trace(dim: int).
3. The new methods are taken into use throughout the code. That should be straightforward for the divergence, but searching for usage of trace operators may require some care.