Add a nice __repr__ to the GPIODevice base class.
This isn't much but generally I think `__repr__` implementations should
be deliberately simple: firstly, they're frequently used for debugging
so if they're at all complex you risk making a debugging tool buggy
(very annoying!). Secondly, if you pour too much info into them you risk
making the debugging output cluttered, so I tend to prefer keeping it to
straight-forward simple to retrieve/calculate info without excessive
detail (if the user wants more, they can always query it directly).
There is one refinement here: in SmoothedInputDevice, `__repr__` is
tweaked to ensure that when partial is False (the default), and the
queue isn't filled, `__repr__` doesn't block (because it should *never*
block).
Fairly major tidy up of the hierarchy as well. There's now a trivial
base class: InputDevice which simply permits reading of state.
WaitableInputDevice descends from this and introduces waitable events
and callbacks, and provides a hook for calling them but needs further
machinery to activate that hook.
DigitalInputDevice (crap name?) descends from WaitableInputDevice and
uses the standard RPi.GPIO callback mechanisms to handle events. This is
intended for use with trivial on/off devices with predictably small
bounce times.
Next is SmoothedInputDevice (crap name?) which also descends from
WaitableInputDevice. This includes a background threaded queue which
constantly monitors the state of the device and provides a running mean
of its state. This is compared to a threshold for determining active /
inactive state. This is intended for use with on/off devices that
"jitter" a lot and for which a running average is therefore appropriate
or for devices which provide an effectively analog readout (like
charging capacitor timings).
MonitorSensor and LightSensor now descend from SmoothedInputDevice, and
Button descends from DigitalInputDevice. All "concrete" classes provide
event aliases appropriate to their function (e.g. when_dark,
when_pressed, etc.)
