Adding const operator wherever it's possible

include/jrl/walkgen/patterngeneratorinterface.hh

...otTrajectoryGeneration/FootTrajectoryGenerationAbstract.h

...TrajectoryGeneration/FootTrajectoryGenerationMultiple.cpp

...otTrajectoryGeneration/FootTrajectoryGenerationMultiple.h

src/Mathematics/Polynome.cpp

src/Mathematics/Polynome.h

src/PatternGeneratorInterfacePrivate.cpp

src/ZMPRefTrajectoryGeneration/OrientationsPreview.cpp

src/ZMPRefTrajectoryGeneration/OrientationsPreview.h

src/patterngeneratorinterfaceprivate.hh