Integrate new kernel method for LFP/ECoG

From the group of Alain Destexhe,

http://cns.iaf.cnrs-gif.fr/abstracts/Kernel2020.html

This should be implemented as a new monitor and supporting documentation/demo.

Ideally, perhaps with help from MNE Python project, we should have a realistic ECoG sensor set with gain precomputed with OpenMEEG.