dynamicgraph::sot is now a singleton

doc/additionalDoc/package.h

include/sot/core/pool.hh

src/factory/pool.cpp

src/feature/feature-abstract.cpp

src/signal/signal-cast.cpp

src/sot/sot-command.h

src/sot/sot-qr.cpp

src/sot/sot.cpp

src/task/task-abstract.cpp

src/task/task-command.h

src/task/task.cpp