Two new y-transformation approaches

[mlindauer]

• bilog (log transformations above 0 and below 0)
• Gaussian Copula (ECDF -> quantiles -> Inverse Gaussian CDF)

[dengdifan]

If everyone is happy with the implementation, I will merge this branch

[alexandertornede]

We will have a look at how these methods perform once we have the new benchmarking fully in place.

[mfeurer]

The recent HEBO suggests using a PowerTransform from scikit-learn. If you plan to benchmark these two, could you also throw this one in the mix?

[alexandertornede]

Thanks for the pointer. Sure!