WIP: Subtract Evoked from Epochs

Add the option to easily subtract an evoked response from each epoch. May be useful for frequency and connectivity analysis when one only want to look at the activity that is not evoked.

This currently doesn't work because when the evoked response is computed, some channels (stim, eog, ecg, etc.) are removed, so the evoked object always has missing channels. If this functionality is considered useful, we can try to figure out a way to support it. E.g., the evoked response is only subtracted from channels that are in both, with an exception if missing channels are data channels, warning for other channel types.