Define a specific type for signal time.

include/dynamic-graph/entity.h

include/dynamic-graph/fwd.hh

include/dynamic-graph/pool.h

include/dynamic-graph/signal-array.h

include/dynamic-graph/signal-helper.h

include/dynamic-graph/tracer-real-time.h

include/dynamic-graph/tracer.h

src/dgraph/entity.cpp

src/dgraph/pool.cpp

src/signal/signal-array.cpp

src/traces/tracer-real-time.cpp

src/traces/tracer.cpp

tests/debug-real-time-tracer.cpp

tests/debug-tracer.cpp

tests/entity.cpp

tests/pool.cpp