Add Fermi distribution function

[lemmatum]

The Fermi distribution function is useful for dense plasmas.

Also there is Fermi-like distribution function which includes chemical potential energy for dense plasmas, which is often used for X-ray Thomson scattering. This is described in Theoretical model of x-ray scattering as a dense matter probe - G Gregori 2003.

[kamalesh0406]

@lemmatum @namurphy I will take up this issue and work on it.

[StanczakDominik]

Awesome! Looking forward to seeing what you come up with @kamalesh0406 :)

[shilpi1958]

Hi, I would like to work on this.
From what I found, I have to write the following function

Fermi-dirac velocity distribution in distributions.py

V is the thermal velocity. Shall I go ahead?

[namurphy]

@shilpi1958 - that would be great, thank you!