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.

src/maxwell/maxwell.F90

@@ -346,7 +346,7 @@ contains
     type is (propagator_exp_mid_t)
       depth = 2
     type is (propagator_leapfrog_t)
-      depth = 1
+      depth = 2
     class default
       message(1) = "Propagator does not yet support initializing of interaction interpolation"
       call messages_fatal(1)