[doc] Add support for custom code examples

This commit adds 2 features to the documentation generation tool:
- components can use one or several 'example' blocks in the docstring
  to provide custom examples. For instance:

  """
  My nice component...
  gngngn

  .. example::
        from morse.builder import *
        toto = Toto()
        ...

  gngngng
  Blabla
  """

  Such examples are automatically moved to the 'Examples' section.

- it also add the :noautoexample: role to prevent the automatic
  generation of a code sample (which sometimes does not make sense
  at all). This role can be used anywhere in the module docstring.

These changes are demonstrated on the ArmatureSensor.
[sensors] Make the GPS more realistic

This commit introduces three level of realism for GPS:

- The default level still returns the (x, y, z) position of the sensor,
  in the blender world.
- The 'raw' level returns geodesic coordinates and linear velocity of
  the robot.
- Last, the 'extended' extends the 'raw' level by providing also some
  human representation for date and hour, and the 'theta' of the sensor.
[sensor] Fix time source used to compute time and date in GPS

The GPS must not use robot.gettime() as this one potientally include
some drift compared to real clock. It must use directly the real
simulation clock.
[doc] Mention GPS improvement