Refactor regridding

Description

Refactor the regridding routines. Create a new type that sets up the mapping between the grids upon initialization. Provide a routine to do the transfer. Use that in various places in the code. Also, fix a bug related to rounding (use nint instead of int as that always rounds down).

As a next step, support for different parallelizations and spacings can be implemented in just one place.

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.