Batchify Maxwell code below exponential_mxll_apply

Description

Batchify the Maxwell code in the Hamiltonian, so below exponential_apply. This is mostly independent of the part above exponential_apply. The code includes the Hamiltonian for the different cases (Faraday-Ampere/Gauss, with/without PML, with/without medium). As a first step, it is implemented for batches on the CPU (packed/unpacked).

For batches on the GPU, it uses the old version combined with get_state and set_state.

News snippet

Batchify Maxwell code below exponential

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.

Closes #405 (closed)