Change kpoints error message to warning in some cases

Description

Up to now, a fatal error is generated when the KPointsGrid variable has a different dimension than needed. However, this should be treated similarly to LSize etc, where the remaining quantities are ignored if there are more (e.g. when specifying 3 instead of 2 for 2D). This allows to use the same input file for runs in different dimensions.

Checklist

• I have checked that my code follows the Octopus coding standards