Ions

Watanyu found some bugs in the ions implementation that even prevented the code from running. I've fixed them, and now the example should run.

• Bugfixes with beam_ion_elements.py
• renamed particles_electromagnetic_fields.py into emfields.py and changed all the imports
• I've added a flag to MPI parallelization that should prevent print statements from appearing for every rank of MPI.

We should think about making a new release or at least of publishing the bugfix to the stable branch :)