Create a general smearing morph

[Sparks29032]

Problem

Generalize the smear morph to not just Gaussians.

Solution

Let's add an option --smear-function or something that can be either a word (e.g. "gaussian", "lorentzian", etc.) or a function in the case of the Python API.

[Sparks29032]

Moved to 0.4.0 release.

[sbillinge]

I moved to backburner. Let's wait for a request.

[Sparks29032]

I am requesting this for comparing DOS curves at different temperatures (Lorentzian broadening).

[sbillinge]

I moved it back to 0.5.0 release. Do you want to do it @Sparks29032 or shall we give it to one of the new people?