analog_read() for Wristband GPIOs

Continuing off from #31 (closed), we could add support for analog_read() from the wristband GPIOs using the ADC peripheral (ref lib/sdk/Documentation/html/group__adc.html).

added 1-Additional-Feature 3-Module labels

I've been thinking about this today while touching gpio.c... Should this live under the same API? API-wise it might be intuitive to:

gpio.set_mode(gpio.WRISTBAND_1, gpio.mode.INPUT | gpio.mode.ANALOG)
gpio.read(gpio.WRISTBAND_1)

but I'm still unsure about this. Symmetry-wise, I like this. My first thought was that this might be problematic regarding the return value type of epic_gpio_read but actually it totally makes sense to keep the "genative return for error" paradigm there, as well since positive-only values do make sense for ADC input, we'd just have to define and document a scaling for the integer values then.

Coming back to symmetry, some might then expect

gpio.set_mode(gpio.WRISTBAND_1, gpio.mode.INPUT | gpio.mode.ANALOG)

to work as well, but they'll either be greeted with -EPARM then. Or they'll be greeted by an epic PWM signal on their wristband

urgs... the 2nd example should've been

gpio.set_mode(gpio.WRISTBAND_1, gpio.mode.OUTPUT | gpio.mode.ANALOG)

of course

mentioned in merge request !229 (merged)

closed via merge request !229 (merged)

mentioned in commit 490ffdc3