[CImg] remove sys calls functionalities
[CImg] remove sys calls functionalities

(remove calls to gm if cimg cannot open a file)

hdeling/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa