add the inverse function of incomplete gamma

[argriffing]

Returns x such that incomplete_gamma(a, x) = y. It could be implemented by searching for the only root of a monotonic function on (0, infinity).

see gammaincinv

[fredrik-johansson]

Complex variables are probably not worth the effort, so this would be a good idea to do after #149

[albinahlback]

I am gonna bump this badboy, this is a good function to have. This is the paper they used: https://dl.acm.org/doi/10.1145/22721.23109.

Edit: Might have to use it this summer, in which case I will try to implement it.