After a year of negotiations, Baystate Franklin Medical Center and its nurses union have reached a tentative new contract. The three-year agreement includes an average 6.2 percent pay increase in the first year, 13 percent over the full contract, and additional staffing in the emergency department and mental health unit. It also keeps current nurse-patient ratios in place. The deal still needs approval from union members, with a ratification vote expected in the coming days.