Clean up the `mesh_type` property of the Grid

[erikvansebille]

In the current v4-dev, the _mesh_type is a property of the Field, but we mostly need access to it in the Grid. A Field requires a UnitConverter, but may not need the mesh information directly.

Hence, we should probably clean up the code so that

1. A Grid has a _mesh_type
2. The UnitConverter of a Field is determined based on both the _mesh_type of its grid and and its name (or another way to find out if a field contains velocity or diffusion values)

[erikvansebille]

Fixed in #2155