Add two variants of the KCI test

[OliverSchacht]

I am adding two variants of the kernel-based conditional independence test, that are aiming at improved computational efficiency.

Namely:

• The Randomized Conditional Independence Test (RCIT) translated from this R code.
• The Fast and Parallel Kernel-based Conditional Independence test (FastKCI), implemented based on an idea of @Biwei-Huang and myself.

I also added two easy unit tests checking the implementation.
I added the algorithms to the cit class, so they can be used with the PC algorithm for causal discovery.

Happy to hear your feedback!

Edit: I think I got something with the signature wrong here... followed the guide but I think something is wrong.

[kunwuz]

Hi Oliver, thanks for these implementations—they're really interesting and helpful! If possible, could you create a new PR or modify this one to include only the files with changes? It seems there's an issue with the commit history. You can ignore the signature; I can handle that manually.

[OliverSchacht]

Hi!

Thanks for the feedback. Yes, I think I messed everything up by trying to rebase and sign. Never worked with that before...
I will make a clean branch with only my additions and create a new PR.