Add general purpose ADC API

ADC1 is configured to sample channels 0 to 15 continuously
at about 30kHz. Users can now set their GPIO of choice to
analog mode (as long as it's wired to one of the analog
channels) and then read the voltage at any time.

The values can be read like this:

GPIO_set_to_analog(GPIO_3_GPIO_Port, GPIO_3_Pin);
my_voltage = get_adc_voltage(GPIO_3_GPIO_Port, GPIO_3_Pin);

Firmware/MotorControl/main.cpp

@@ -81,6 +81,9 @@ int odrive_main(void) {
                 *encoder, *sensorless_estimator, *controller, *motor);
     }
     
+    // Start ADC for temperature measurements and user measurements
+    start_general_purpose_adc();
+
     // TODO: make dynamically reconfigurable
 #if HW_VERSION_MAJOR == 3 && HW_VERSION_MINOR >= 3
     if (board_config.enable_uart) {