Change namespace Vector6d to dynamic_graph::sot

[NoelieRamuzat]

Dear all,

Current pb: we have moved a part of vector-conversions.hh to matrix-geometry.hh to share the tools.
The problem is that matrix-geometry.hh use to have Vector6d in the namespace dynamic_graph::sot.
The trick in vector-conversions.hh of using the namespace of Eigen might be misleading (actually it was for Olivier) in letting think the reader that this is an Eigen type.
If this is ok with you I would advocate to use the namespace dynamic_graph::sot with a short cut (dyn::sot) for Vector6d, and propose a PR on sot-torque-control ?

Thank you
Olivier and Noëlie