SpikeTrain should allow direct access to the channel from which it was recorded

[apdavison]

[JuliaSprenger]

This could be constructed on request when loading a structure containing groups that are flagged as linking spiketrains to corresponding recording traces (channelviews). This requires introducing a group type for 'pairing' data objects. This should be done only on demand for performance reasons. This can be generalized also to multidimensional channelview objects to link a spiketrain list and multiple recording traces of a signal.