Allow different predictor-values per channel

[behinger]

It would be great to have different predictor values on a per-channel basis. The application would be to have channel-wise baseline-regression ala Alday et al.
E.g. if you use a formula y~1+effect+bsl then you'd want the bsl to be specific on the channel you currently fitting.

This is currently not possible, one idea would be to have a custom solver, that injects the correct baseline values into the designmatrix.