Fix a problem in compiling unit_PYTHON (the wrapper for the core library).
Merge branch 'develop' of https://github.com/projectchrono/chrono into develop
Having moved the static std::string irrlicht_default_obj_dir into the ChIrrAppInterface caused the SWIG python wrapper generator to fail.
Tho fix this, I moved back such static  irrlicht_default_obj_dir  in the original place (the ChBodySceneNodeTools.h), that was never processed by SWIG and so the SWIG error is gone.
Note, by the way, that such static variable was  used only by functions in ChBodySceneNodeTools, plus in the ChIrrAppInterface to define the directory for fonts.
Since the use of static vars is not a good practice (exp. here in a .h, that might result in multiple static data per each .cpp object), for defining font directory we now introduced a SetFonts() function in ChIrrAppInterface, and the ChBodySceneNodeTools is kept here in its original form only for backward compatibility (but will removed in future, anyway, because  now we have the ChAsset - based visualization system)