added lorentzian function

[thissop]

Hello!

This library is very helpful, I added a lorentzian function to the periodic fit capability because I use this for modeling peaks in power density spectra of black hole light curves, it might be useful for other areas as well :)

http://emilygraceripka.com/blog/16

[thissop]

example in use:

[TomDonoghue]

@thissop - this looks fantastic, thank you for the submission!

Quick Q: would you have any issue with us potentially calling this a 'cauchy' fit function, as I understand it is sometimes also called? In the context of the neural data, we sometimes call the 1/(f**x + k) a 'lorentzian function - so calling this function one as well would likely get a bit confusing!

Also - this is actually great timing as we are currently building in additional fit functions with the ongoing 2.0 developments. In general, we would want to add some test and examples of this fitting - if you have any strong desire to contribute those that's great, but since I'm currently working on related update, if it works for you I can also just merge this directly in, and then add some tests (& maybe a name change, etc) as I work through things from here!