In a state that prides itself on its medical and scientific prowess, UMass Medical School is perhaps its best-kept secret. Established by the legislature just 50 years ago, the school is undergoing a growth spurt that has been little noticed by the general public but has caught the attention of people at the cutting edge of scientific research. The Worcester campus is adding 100 faculty to fill a $400 million biomedical research building set to open this year.