Decorate ion_sound_speed with @particle_input

[namurphy]

This PR decorates the plasmapy.formulary.speeds.ion_sound_speed with @particle_input in order to enhance compatibility with ParticleList and CustomParticle objects. The z_mean parameter for ion_sound_speed has been deprecated.

[github-actions]

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (aff21ae) 98.38% compared to head (652d53e) 98.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2134      +/-   ##
==========================================
- Coverage   98.38%   98.38%   -0.01%     
==========================================
  Files          98       98              
  Lines        8625     8624       -1     
==========================================
- Hits         8486     8485       -1     
  Misses        139      139              

Impacted Files	Coverage Δ
plasmapy/dispersion/analytical/two_fluid_.py	100.00% <ø> (ø)
plasmapy/dispersion/numerical/hollweg_.py	100.00% <ø> (ø)
plasmapy/dispersion/numerical/kinetic_alfven_.py	100.00% <ø> (ø)
plasmapy/formulary/speeds.py	97.77% <100.00%> (-0.03%)	⬇️

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[ejohnson-96]

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