Add command clear in Task and fix bug in FeaturePoint6dRelative

  Signal errordotSOUT was created twice. This raised an exception
  at instance creation.

  Entity names with dashes make dot crash when translating the graph into pdf.
  Put names into double quotes.

  These symbols have been moved to sot-tools.

  - replace commande setPosture by input signal posture.
  - dimension of feature is equal to number of active dofs.

Conflicts:
	src/dynamic_graph/sot/core/__init__.py

  Signal velocitySIN has been added to provide the reference velocity of the
  feature. This signal is used to compute the error time derivative produced by
  the reference velocity, i.e. the variation of the error if no correction is
  done.

  \dot{e} &=& J\dot{q} + \frac{\partial e}{\partial t}\\
  \dot{q} &=& J^{+} (\dot{e}^{*} - \frac{\partial e}{\partial t})\\
          &=& J^{+} (-\lambda e - \frac{\partial e}{\partial t})

  - some assertions seem not to ever have been compiled in debug mode.

  - At entity construction, x_est and P depend on no signals
  - when initializing the entity with initial state estimation and variance,
  - signal x_est is set to this initial values and signal dependencies are
    set.

  Implement extended Kalman filter: user should provide
    - predicted state,
    - predicted observation,
    - partial derivative of state transition,
    - partial derivation of observation function
  through input signals.

  Implement python bindings.

  in -> sin since "in" is a key word in python language,
  out -> sout for consistency.

  - encapsulate into namespace dynamicgraph::sot,
  - define commands to set and get parameters,
  - empty commandline method.