Introduce a method to switch between built-in and headset microphone depending on the presence of something plugged into the 3.5mm headphone jack, included notes regarding measurements, simulations, and various findings for the headset/built-in mic opperation

Zener diode included to protect the GPIO pin against under/over voltage from HP_OUT. Schmitt-trigger inverted at the enable pin of the analog switch for some debouncing.