change main Makefile to use -C option while cleaning
change main Makefile to use -C option while cleaning

wkrzemien/j-pet-framework,alexkernphysiker/j-pet-framework,wkrzemien/j-pet-framework,JPETTomography/j-pet-framework,wkrzemien/j-pet-framework,alexkernphysiker/j-pet-framework,alexkernphysiker/j-pet-framework,JPETTomography/j-pet-framework,alexkernphysiker/j-pet-framework,wkrzemien/j-pet-framework,alexkernphysiker/j-pet-framework,JPETTomography/j-pet-framework,wkrzemien/j-pet-framework