Add checks for duplicate channels and support for appending to an existing ElectrodeTable

[stephprince]

Currently, the API assumes all electrodes have been added and the user cannot add additional channels after the initial ElectrodeTable initialization. To support cases where multiple types of datasets (e.g. ElectricalSeries, SpikeEventSeries) are created from different subsets of electrodes on the ElectrodesTable, we should add support to append additional channels to an existing ElectrodeTable.

To do this, we will need to add checks for duplicate channels. A proposed approach is to:

1. In ElectrodeTable->addElectrodes(), add a check using the global index/rowIds to only add channels not yet included
2. In ElectrodeTable->finalize(), if the columns already exist and we have all the data for all of the columns (including custom columns), use an addRow method to append additional channels, otherwise create the columns with all the data.

[stephprince]

Related to: #13