Add a clean for DBHandler in the main Makefile.
Add a clean for DBHandler in the main Makefile.

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