ENH: Added TrialHandler support to ioDataStore

Standard psychopy TrialHandler's can now be used to create a table in
the ioDataStore which can hold the values of trial rows as they are used
during the experiment. See
psychopy.iohub.client.ioHubConnection.createTrialHandlerRecordTable
method comments for usage example.

The ioHub ExperimentVariableProvider class should likely be deprecated
in favour of using psychopy's standard TrialHandler.