cartesian_to_depth_coordinates utilities in the geometry model

[lhy11009]

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A new geometry model interface function to convert Cartesian positions
to depth coordinates while preserving geometry-specific lateral coordinates.
The box geometry now returns depth together with the horizontal Cartesian
coordinates.

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[lhy11009]

There is currently an option for the "Coordinate system" as depth, which allows only one entry into the function as depth lateral coordinates (e.g. x in box). This is largely restricted by this function in source/geometry_model/interface.cc:

Interface<dim>::cartesian_to_other_coordinates

and the fact that it uses:

           case Utilities::Coordinates::depth:
             other_coord[0] = depth(position);

My proposal of change is to largely keep the current interface, while making a virtual function of Interface<dim>::cartesian_to_depth_coordinates
This would do the exact thing of providing only the depth for the base of the Interface class, while taking different implementations for the daughter classes like Box.

For instance, I implement the version for box geometry.

Given the current change, I am not sure what a good strategy is for a test case. I have only tested in the models I run.