This finishes off implementing values and source for all (current)
classes in gpiozero. I'm afraid things get rather complex in this
commit. For starters, we've now got quite a few "aggregate" classes
which necessarily don't descend from GPIODevice. To implement values and
source on these I could either repeat a helluva lot of code or ... turn
to mixin classes. Yeah, it's multiple inheritance time, baby!
Unfortunately multiple inheritance doesn't work with __slots__ but we
really ought to keep functionality that they provide us (raise
AttributeError when an unknown attribute is set). So I've implemented
this with ... erm ... metaclasses. Sorry!
Permit devices to be explicitly closed and used as context managers.
Also deal with cleanup properly at script end and ensure objects don't
step on the global cleanup function.
The motion sensor queue doesn't shut down properly at script end at the
moment and prevents the interpreter shutting down. This is because it's
a non-daemon thread so `__del__` never gets run and so on.
This is a bit of a major PR - I can split it up if you want. Firstly it
makes a common base class called `GPIODevice` for both `InputDevice` and
`OutputDevice`. This just takes care of the read-only pin stuff. Next it
makes a `GPIOThread` class that ensures its a daemon thread, and which
also ensures proper cleanup on shutdown.
Finally, it fixes `MotionSensor` to use the new `GPIOThread` class
(tested this time! Works nicely) and adds the `blink` method to the
`LED` class (which also works nicely this time).
Small patch to ensure GPIO.cleanup is always called on interpreter
shutdown. This just means the library will "play nice" with whatever
comes afterward.
