[nabobalis]

Avoid repeated expensive recomputation in the level populations calculation while keeping the output bit-identical:

• Cache the Ion instances returned by next_ion()/previous_ion() for internal use so that rate matrices of the recombining ion are only computed once rather than on every density iteration.
• Cache the density-independent sum of the two-ion rate matrices.
• Memoize plasmapy atomic_number/atomic_symbol/element_name lookups, which construct an expensive Particle object on every call.
• Share the proton-electron ratio with the next ion since it only depends on temperature and the abundance/ionization datasets.

Benchmark (21 temperatures x 9 densities): Fe XXIII 73.5->7.2 s, Ca XVIII 74.3->8.1 s, O VI 73.3->9.3 s, Fe XII (single-ion model) 9.9->5.0 s, coupled n-T 14.5->3.0 s.