The IntanThread can report TTL events on its 6 input channels. This was tested
with an Arduino; input detection is fast and reliable.

The IntanThread also generates its own timestamps, allowing the software to move
toward a framework in which timestamps come from input sources, rather than
from the RecordNode.

Finally, an ArduinoOutput module was added, which allows the software to
communicate with an Arduino via serial output.

Changes to the DataThread, SourceNode, and UIComponent classes allow data
threads to disable callbacks if they are no longer receiving input. So
far this has only been tested with the Intan Board, but it works really well.
Unplugging the board in the middle of acquisition allows the thread to exit,
and callbacks to be disabled, without any seg faults.

DataThreads (e.g. IntanThread, FileReaderThread, FPGAThread) are now created at
the same time as the source node, rather than at the start of data acquisition.
New methods for starting/stopping individual threads are required, although
only the appropriate methods for the IntanThread have been written.

Another important change is that the SourceNode now periodically checks for
an appropriate input source every few seconds while acquisition is not in
progress. It's the responsibility of the individual DataThreads to notify
the SourceNode if their input has disappears. In the case of the IntanThread,
this involves attempting to change the baud rate. If an error code returns,
it informs the SourceNode that the input is missing. This, in turn, informs
the FilterViewport that the source is no longer enabled, thus deactivating
that particular signal chain.