Discussion: Idea for refactoring parts of grid

As a part of (or replacement of) some points in #556, methods calculating various properties related to the geometry of the Grid class (ie. not belonging to actnum or the grids properties) could be separated into using a public facing interface. In #640 a design idea for an abstract class, implemented by Grid (and possibly also directly by `RoffGrid` and `EclGrid`) through which several methods of Grid can be replaced (for instance `get_cell_volume`, `get_xyz_cell_corners`, `get_xyz_corners`, `get_layer_slice`, `get_geometrics`, `get_randomline`, `get_dz`, etc.) by a function which the interface knows nothing about.

This means more flexibility for the user, and less implementation details for us.

Start by reading discussions #556 and #640, then review the listed Grid geometry methods: get_cell_volume, get_xyz_cell_corners, get_xyz_corners, get_layer_slice, get_geometrics, get_randomline, and get_dz. The issue does not define the interface or a concrete completion boundary; done would require an agreed design and corresponding separation of these methods from Grid implementation details.

Written by the indexing model from the issue text.