Move doxygen comments of public declarations from cpp to header files

Created by: ypiguet-epfl

At some places such as in aseba/compiler/compiler.cpp for method compile, the doxygen documentation of public methods is located in the cpp file. It would be better to put it in the header file (aseba/compiler/compiler.h), because the cpp file could be provided as compiled code and not readily available. That's already the case for some of the public declarations (e.g. struct SubroutineDescriptor).