Add latex_str attribute to Particle

[namurphy]

We should add a Particle.latex_str attribute to have a LaTeX representation of particle symbols. For example, Particle("p") could return something like r"p$^+$".

We should make sure that we do this consistently with other latex_str attributes in PlasmaPy, such as for the fit functions.

This issue is partially a prerequisite for #2639.

We might consider adding a CustomParticle.latex_str in there too, probably defaulting to CustomParticle.symbol, whilst letting it be overridden by users.