Openpmd-beamphysics

- Add `pmd_beamphysics.interfaces.astra.parse_astra_phase_file` (this came from LUME-Astra)
- This extends species handling to protons, positrons, hydrogen

- update the name in `setup.py` to `openpmd-beamphysics`, which matches the conda-forge name.
- add convenience function `particles.centroid`. Note that this is *not* a method in the ParticleGroup class.

New `pmd_beamphysics.labels` with two functions to get a math symbol from a key:
- `texlabel(key)` returns a mathtext (TeX) string.
- `mathlabel(*keys, units=units, tex=True)` returns a label with optional units and including the `$...$` for use in Matplotlib labels.

An example notebook `examples/labels.ipynb` demonstrates the usage.

- correct `interfaces/impact.py` cathode emission

- added versioneer

Initial version for PyPI