allow choosing the center meridian for cell_centers and cell_boundaries

[keewis]

For the cell boundaries the longitude is currently always returned in a range of -180°-180°, while for the cell centers this depends on the dggs implementation library.

I think the defaults should match, and it should be possible to choose a different range when necessary. For now I would limit that choice to 0° and 180°, as those are the most common, and having a discrete set of meridians makes it much easier to implement.