Head docstring clean-up

__init__.py

impedance/__init__.py

impedance/csr.py

impedance/impedance_model.py

impedance/resistive_wall.py

impedance/resonator.py

impedance/tapers.py

impedance/wakefield.py

instability/__init__.py

instability/instabilities.py

instability/ions.py

tracking/__init__.py

tracking/aperture.py

tracking/element.py

tracking/monitors/__init__.py

tracking/monitors/monitors.py

tracking/monitors/plotting.py

tracking/monitors/tools.py

tracking/parallel.py

tracking/particles.py