Adds Beam.init_bunch_list_mpi

Beam.update_filling_pattern and Beam.update_distance_between_bunches are now called directly in the __init__ when a bunch_list is given.
Beam.init_bunch_list_mpi is a convenience method to initialize a beam using MPI parallelisation with a Bunch per core.
Add tests for Beam.init_bunch_list_mpi