add option to include source_channels for DecompositionSeries

[bendichter]

include round trip test

Motivation

pynwb changes corresponding to NeurodataWithoutBorders/nwb-schema#450

How to test the behavior?

self.make_electrode_table(self)
region = DynamicTableRegion(name='source_channels',
                            data=[0, 2],
                            description='the first and third electrodes',
                            table=self.table)
data = np.random.randn(100, 2, 30)
timestamps = np.arange(100)/100
ds = DecompositionSeries(name='test_DS',
                         data=data,
                         source_channels=region,
                         timestamps=timestamps,
                         metric='amplitude')

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[bendichter]

tests pass with NeurodataWithoutBorders/nwb-schema#450