Multichannel math

[HenryThinks]

Some otherwise good recordings can be obscured by reference noise; particularly movement artefact. Multichannel recordings offer a route to this sort of noise subtraction using signal averages. While the signal modification could be performed elsewhere it would be lovely to have it as an option under the 'Plot controls', allowing a quick assay of its utility in a particular recording. Likewise, noise subtracted signals might also be useful in feature extraction.

Otherwise great work and I'm loving the project!