Encapsulate upscaling with a high-level manager

This manager class is over time expected to take every input part
(grid, two-phase and rock parameters, wells) of a three-dimensional
case and transform them into two-dimensional counter-parts.

On purpose it has a very minimal interface, exposing only necessary
functionality to the user code, whereas most of the implementation
is hidden away in the translation module. This allows for tweaks and
changes without encountering massive recompiles.