Make boris the default bunch pusher and enable parallelism

[AngelFP]

• Make 'boris' the default pusher for the particle bunches.
• Raise a ValueError if the given bunch_pusher is not recognized.
• Faster particle gathering thanks to a more efficient method for resetting field arrays.
• Enable bunch pusher, bunch field gathering and analytical fields to run in parallel (other functions like those for bunch deposition are still serial. This will be gradually implemented).
• Define a new environment variable 'WAKET_NUM_THREADS' that controls the number of parallel threads (by default, 1).