Use linear interpolation for the leapfrog propagator for Maxwell

Description

For this, we need a depth of 2 for the interpolation in time. Otherwise, it is using constant interpolation, reducing the order of the scheme to one.

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.

changed milestone to %14.0

added Bugfix label

assigned to @sohlmann

approved this merge request

merged

mentioned in commit 5b84ea0b

added in Changelog label