Vectorized implementations of 3/4 partner selection approaches

[franz101]

Coded mostly in numpy, pandas and scipy. I have focused on different strategies to improve the speed further.

• Avoid for-loops as much as possible
• Use broadcasting and matrix multiplication instead
• Experiment with advanced indexing and one hot encoded matrix multiplication

In this PR the following is included:

• The base structure for a module that can be used for partner selection.
• A stock downloader which is build on top of y-finance
• A notebook with a tutorial on how to use it.

Outlook:

• I have not yet finished the vectorised copula approach.
• I wanted to build a few dashboards outside of Python
• Unit testing is still behind
• An architecture to make it more usable with static methods for example.

If you have any questions. I'm looking forward to discuss them.