standard matrix slicing not supported for DataPipe object

[cechava]

Currently, accessing data in an NWB file with standard matrix requires reference to the DataStub object. For example:

nwb.acquisition.get('TwoPhotonSeries').data.internal.stub(:, :, 1:2000);

It would be more practical if the data could be accessed in this way without having to refer to the subfields of the TwoPhotonSeries object. For example,

nwb.acquisition.get('TwoPhotonSeries').data(:, :, 1:200);

This might require extending the functionality of the DataPipe class