MaxwellIO: Fix issue due to disparity between routed and mapped channels

[LeMuellerGuy]

As mentioned in #1702, there can be a potential disparity between the list of mapped channels and routed channels when using stimulation electrodes. This PR attempts to fix this by ignoring the additional electrodes since they have no data associated to them/ neo can currently only pull data from the routed group. I don't know if there can be additional groups in the data though.