[sensor] Semantic Camera 'tag' and 'relative' properties

- 'tag' allows to select the kind of object visible
by the semantic camera
- 'relative' returns the visible objects position/orientation
in the camera frame

Fix #496
[doc] Document new features of semantic cam
[builder] Remove the VideoCamera::rotate overload

As discussed in #399.
Revert "[builder] Remove the VideoCamera::rotate overload"

This reverts commit f0bcf5114116cf2417a70337b35f0ffe5074dd08.

In the current state, it is completely incorrect, and breaks all
unit-test. Will rework it later in #500.
[middleware/pocolibs] Check properly bound in velodyne

Fix #508
While here, report properly errors with PyErr_* API
[pymorse] fix connect ebadf due to fast thread loop
[doc] Mention the specific camera frame
[builder] fix bpymorse.set_viewport_perspective
[pymorse] fix missing chunk in asynchat patch
[builder] Set ControlType to Position for PR2 robot

Reported by Omar Adair Islas Ramírez
[doc] Fix the ControlType for XYW Motion for PR2

Reported by Omar Adair Islas Ramírez
[testing] Relax precision on YARP tests

It always was less precise than socket one, but was not properly bumped
when the controller changes (to use proper physics)
[builder] Set the gravity in a consistent way

Gravity is stored in multiples place in Blender, set "all" of them.
Make sure the default is 9.81, as written in the documentation.

Fix #514
[builder] Make sure the laser are not rendered

Previously, they can appear in a videocamera fov
[sensor/camera] Handle more cleverly relations between objs of different scenes

Contrarily to what we can expect, several objects can have the same name
in the same scene. So we need to track the relation between the objects
of the different scene in a smarter way. As said in the code comment,
I'm not completely sure it is totally correct, but it largely improve the
situation, and I can't see any regression.

Should fix #509
[time] Update the timestamp from ms to s

Fix #498