WIP: Resolve "Enable OpenMP for mesh_batch_nrm2"

Description

The current implementation for mesh_batch_nrm2 does not lend itself for OpenMP parallelization. As none of the extra measures are taken for the UNPACKED and DEVICE_PACKED versions, it might be worth reverting to a simple dot product which can be parallelized.

News snippet

Enable OpenMP for mesh_batch_nrm2.

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 #328 (closed)