[builder] add Robot.set_friction and test
[robots] Document in the code properly each robot

Last step for #284
[doc] Don't print data section for robots
[doc] Add link to set_mass and set_friction in the robot documentation
[helpers] Factorise the code used for add_{level, data, property}
[doc] Recursively get level, data, properties information

While here, factorize the code.
[multi] new morse multi-node daemon (basic socket)
[test] fix missing '_testing' in CMakeLists
[builder] fix #421 multi WheeledRobot
[tests] Use properly morse.testing.testing.main
[core] Don't use class variable where instance variable are needed

Magically fix #422
[multi] using Steam.connected instead of is_up