Add an example to read VBUS and VSYS (#331)

The process is different on Pico and Pico W so demonstrate how to do it.

Fixes #324

adc/read_vsys/power_status.h

@@ -0,0 +1,33 @@
+/**
+ * Copyright (c) 2023 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef POWER_STATUS_H
+#define POWER_STATUS_H
+
+/*!
+ * \brief Get power source
+ *
+ * Returns whether battery powered
+ * \note On Pico W must have called cyw43_arch_init
+ *
+ * \param battery_powered True if powered by battery, False if powered by USB or another means
+ * \return Zero if the battery status can be determined, an error code otherwise \see pico_error_codes
+ */
+
+int power_source(bool *battery_powered);
+
+/*!
+ * \brief Get system voltage
+ *
+ * Returns the system voltage
+ * \note Must have called adc_init
+ *
+ * \param voltage Calculated voltage
+ * \return Zero if the voltage can be determined, an error code otherwise \see pico_error_codes
+ */
+int power_voltage(float *voltage);
+
+#endif