`mathlabel` fails to fix some labels, breaking plots

[ken-lauer]

mathlabel fails to fix some - as of yet unsupported - labels, breaking plots that contain such labels.

This is in the context of LUME-Impact - specifically in my addition of IMPACT-Z support:

\mathrm{ loadbalance_min_n_particle }
\mathrm{ loadbalance_max_n_particle }
\mathrm{ charge_state_n_particle }
\mathrm{ neg_cov_z__gammabeta_z }
\mathrm{ neg_cov_y__gammabeta_y }~(\mathrm{ m } )
\mathrm{ neg_cov_x__gammabeta_x }~(\mathrm{ m } )
\mathrm{ neg_cov_x__gammabeta_x }~(\mathrm{ m } )

I'll special-case them in LUME-Impact for the time being, but could something general be done in openPMD-beamphysics instead?