Investigation by the Department of Public Utilities on its own motion as to the propriety of the rates and charges set forth in the following tariffs: M.D.P.U. nos. 944 through 970, filed with the Department on May 17, 1996 to become effective June 1, 1996 by Boston Gas Company: and investigation of the proposal of Boston Gas Company to implement performance-based ratemaking, and plan to exit the merchant function