Investigate using Riemannian geometry

[ErikBjare]

@alexandrebarachant, who authored pyRiemann, has won several competitions with Riemannian approaches:

• https://github.com/alexandrebarachant/Grasp-and-lift-EEG-challenge
• https://github.com/alexandrebarachant/decoding-brain-challenge-2016
• https://github.com/alexandrebarachant/bci-challenge-ner-2015

[ErikBjare]

Implemented in cf6b56b