Add support for inverse function

[dominikue]

Hi, for our project we adjusted the code to use an inverse function. I think this could be helpful for some others. Please let me know if you want a reproducible example or a pull request :)

Adjust line 111 in questplus/psychometric_function.py:

- return p
+ return (1 + lower_asymptote - lapse_rate) - p

Example plot:

[hoechenberger]

Very nice! If you would like to make a pull request, please do go ahead :) Be sure to include some tests too, if possible :)