replace ProcessorFactory class with new Factory typedef

This pr follows !80 (merged) and use the new Factory class.

ProcessorFactory is then a simple typedef.

Add also a macro to ease processor registration & use it in ProcessorIMU as an example.

src/CMakeLists.txt

@@ -217,7 +217,6 @@ SET(SRCS
     node_base.cpp
     problem.cpp
     processor_base.cpp
-    processor_factory.cpp
     processor_imu.cpp
     processor_odom_2D.cpp
     processor_odom_3D.cpp