Plasmapy: Integrate Particle into arguments in langmuir.py

[lemmatum]

Currently, the langmuir.py module uses the gas argument in a number of functions to define the mass of the plasma species. This should be refactored to use the Particle class instead.

EDIT: and of course rename the argument variable to something more descriptive and consistent with the rest of PlasmaPy!

[carolyz]

Hi! Wondering if this is still open and if I could take a crack at it. Thanks!

[StanczakDominik]

I don't recall any work being done towards this. Go right ahead! :)