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.

UIComponent, ProcessorGraph, etc. were still referencing the configuration
class (but not using it). Those references have now been removed, as the
configuration object will be replaced by a set of parameter and channel
objects.

Processors used to have their sample rate and inputs/outputs set when
their source/dest nodes were set. Now, this step doesn't occur until
after the signal chains are constructed. So far, this seems to make
things run much more smoothly.

Since every processor now gets asked to enable() prior to the start of
acquisition, it's much simpler to move the steps involved in preparing
for acquisition into these methods. This the processor from beginning
acquisition when prepareToPlay() is called upon initialization.

The IntanThread was occasionally looking for data after acquisition was disabled,
which would trigger the SourceNode to attempt to manually disable callbacks.
The only negative effect of this was that if the user was currently viewing
a different signal chain, it would switch back to the Intan chain unexpectedly.
This is no longer the case, as the SourceNode checks to see whether or not
it received a proper disable signal from the ProcessorGraph before notifying
the UI that data may have been dropped.

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.