Audit for frequencies.jl : gyrofrequency and plasma_frequency

[Beforerr]

• Item input/output uses Unitful.jl to keep track of units
• Particle quantities can be specified using ChargedParticles.jl, if applicable
• Arguments are order independent, or it is documented clearly that this is not the case
• Output tested against known good examples, either using doctests or in the package tests
• Output tested against plasmapy, if possible.
• Docstring with a description of the physical meaning of the quantity, a doctest example, cross references, external references to e.g. Wikipedia, plasmapy, etc.
• Item is included in the appropriate section of the docs.

[Beforerr]

This CI somehow is tricky. jldoctest in different OS system would give "s⁻¹" or "s^-1".