Add nearest-neighbor interpolation for regridding

Description

This makes the regridding faster (but also less accurate). The behavior can be controled by the variable RegriddingInterpolationLevel.

For an even ratio of the spacings, the point exactly in the middle between two coarse grid points always belongs to the left coarse point (to avoid ambiguities).

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.