Add thermionic emission equations

[lemmatum]

When Langmuir probes get hot, they can emit particles (just like old devices that used hot electrons from a heated wire to generate plasmas). We should add the equation(s) for thermionic emission, especially in the case of hot Langmuir probes.

[github-actions]

This issue will be closed in 90 days due to five years of inactivity unless the stale label or comment is removed.

[lemmatum]

Bump