Batchification of maxwell propagator routine

Description

Batchify the Maxwell propagator routine to avoid batch_{get,set}_state. To this end, the transformation routines are batchified to return batches which are then used for the exponential and for the pml part.

News snippet

Batchify Maxwell propagator routine

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.