api/iodevices: Separate Ev3devSensor class

This is not just an alias for a LUMPDevice because it works differently. On ev3dev, sensor modes are set using strings.

Also, simplify the introductory text and reasons for using it.

doc/api/iodevices_ev3.inc

@@ -77,31 +77,25 @@ This class can be used to control classic LEGO motors using converter cables.
 Ev3dev sensors
 ^^^^^^^^^^^^^^^^^^^^^
 
-EV3 MicroPython is built on top of ev3dev, which means that a particular sensor
-may be supported even if it is not listed in this documentation. To test whether
-your sensor is supported:
+EV3 MicroPython is built on top of ev3dev, which means that a sensor
+may be supported even if it is not listed in this documentation. If so, you can
+use it with the ``Ev3devSensor`` class. This is easier and faster than using
+the custom device classes given above.
+
+To check whether you can use the ``Ev3devSensor`` class:
 
     * Plug the sensor into your EV3 Brick.
     * Go to the main menu of the EV3 Brick.
     * Select `Device Browser` and then `Sensors`.
-    * If your sensor shows up, it is supported.
+    * If your sensor shows up, you can use it.
 
-You will be able to use this sensor using the ``Ev3devSensor`` class below.
-This lets you read the sensor values for a given mode number. To see which modes
-are available for your sensor:
+Now select your sensor from the menu and choose `set mode`. This shows all
+available modes for this sensor. You can use these mode names as the ``mode``
+setting below.
 
-    * Follow the steps above and select your sensor.
-    * Select `set mode`. This shows all available modes for this sensor.
-    * You don't need to set a mode now, so press the back button to cancel.
-
-To read values in your program, enter the mode number as the ``mode`` setting
-in the class shown below. The first mode is mode 0, the second mode is mode 1,
-and so on. To learn more about compatible devices and their modes, visit the
-`ev3dev sensors`_ page.
+To learn more about compatible devices and what each mode does,
+visit the `ev3dev sensors`_ page.
 
 .. _ev3dev sensors: http://docs.ev3dev.org/projects/lego-linux-drivers/en/ev3dev-stretch/sensors.html
 
 .. autoclass:: pybricks.iodevices.Ev3devSensor
-    :no-members:
-
-    .. automethod:: pybricks.iodevices.Ev3devSensor.read

pybricks/iodevices.py

@@ -34,8 +34,27 @@ class LUMPDevice():
         pass
 
 
-class Ev3devSensor(LUMPDevice):
-    pass
+class Ev3devSensor():
+    """Read values with an ev3dev-compatible sensor."""
+
+    def __init__(self, port):
+        """
+
+        Arguments:
+            port (Port): Port to which the device is connected.
+        """
+        pass
+
+    def read(self, mode):
+        """Read values at a given mode.
+
+        Arguments:
+            mode (``str``): Mode name.
+
+        Returns:
+            ``tuple``: Values read from the sensor.
+        """
+        pass
 
 
 class AnalogSensor():