For the past decade, Dr. Joseph A. Grocela has been tinkering in the basement workshop of his Weston home, building inventions ranging from urological devices to a voice box that can help musicians harmonize and improve tonal precision for tone deaf people. Now the urological surgeon has mounted a legal challenge to his employer, Massachusetts General Hospital, and its corporate parent, Boston’s Partners HealthCare System Inc., over who owns the intellectual property rights to the inventions he develops on his own time.