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.