Add documentation for the py power module

Documentation/pycardium/power.rst

+.. py:module:: Power
+
+``power`` - PMIC power module handling
+======================================
+The :py:mod:`power` module allows you to read the card10's power status
+in your scripts.
+
+**Example**:
+
+.. code-block:: python
+
+   import power
+
+   print(power.read_battery_voltage())
+
+.. py:function:: read_battery_voltage()
+
+   Read the battery voltage in V.  Please keep in mind that battery
+   voltage behaves exponentially when interpreting this value.
+
+   .. warning::
+
+      Card10 will hard-shutdown once the voltage drops below 3.4 V
+
+.. py:function:: read_battery_current()
+
+   Read the battery-side current flow in A.
+
+.. py:function:: read_chargin_voltage()
+
+   Read the charge voltage in V.
+
+.. py:function:: read_chargin_current()
+
+   Read the charge current in A.
+
+.. py:function:: read_system_voltage()
+
+   Read the system-side voltate in V.
+
+.. py:function:: read_thermistor_voltage()
+
+   Read the thermistor voltage in V.
+
+   There is a resistor network from GND over a thermistor
+   (10K at room temperature) over 10K to the Thermistor Bias voltage.
+   This reads the voltage between thermistor and resistor.