Improve access of DynamicTable and DynamicTableRegion subset IDs

Given a DynamicTableRegion, e.g., ElectricalSeries.electrodes, to access the IDs of the subset of the table, one needs to do:

electrode_table_indices = eseries.electrodes.data[:]
selected_elect_ids = eseries.electrodes.table.id[electrode_table_indices]

selected_elect_ids = list(eseries.electrodes.to_dataframe().index)

It would be a nicer user interface to allow:

eseries.electrodes['id'][:]
# or
eseries.electrodes[:, 'id']
# or even
eseries.electrodes.id[:]

Also, given a DynamicTable, e.g., NWBFile.electrodes, to access the IDs of the table, one needs to do:

nwbfile.electrodes.id[:]
nwbfile.electrodes['id'][:]

but

nwbfile.electrodes[:, 'id']

does not work, a main inconsistency with how users access data from a column.

Checklist

[x] Have you ensured the feature or change was not already reported ?
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[x] Have you included a clear description of the problem you are trying to solve?
[x] Have you included a minimal code snippet that reproduces the issue you are encountering?
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