Add `cfg.montage` to `preprocessing`

[tensionhead]

Field Trip's ft_preprocessing has a montage cfg parameter, which internally calls ft_apply_montage. This function applies a linear operator (the matrix given in montage.tra) onto the channel axis, creating linear combinations (neighbor subtraction, mixing, ...). For Syncopy this means (for the 1st time?) that we need a StructDict as parameter value for the montage cfg entry, other fields are montage.labelold and montage.labelnew.