add detrendent cross-correlation analysis

[dokato]

eg from: https://www.sciencedirect.com/science/article/pii/S0378437113000332

[LRydin]

Hey @dokato, on my own MFDFA code I was also planning to work on this (check issue 11). In case you are interested, I am happy to give this a go as a joint operation :).

Best,
Leo

[dokato]

@LRydin good job with MFDFA. Sounds good. I gave it first try some time ago. Once I clean my code, can send you over.

[LRydin]

Cool. I haven't dived yet too closely into the mathematics of the problem, but it seems like an interesting project for the future :). Cheers!