Implement generic regridding

Description

The generic scheme works as follows:

• for each point of the output mesh, get the surrounding 8 points of the input mesh
• determine the interpolation coefficients
• for the regridding itself, loop over those 8 points

This leads to a scheme that is not adjoint when used for 2 grids in forward and backward direction, but at least it is a first implementation that works.

News snippet

Implement generic regridding

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.