Dppvfx dev

[danielecc]

Implementation of the associated paper at NeurIPS '19 on very fast and exact DPP sampling with intermediate sampling and Nystrom approximation.

[guilgautier]

Hi @danielecc,

Thanks for this very interesting PR!

Let's continue working in this branch to prepare integration into DPPy.

In particular, for the many different functionalities one must write

• tests
• docstrings
• documentation (here is the .rst) to explain the method