Mixture properties of the water + .gamma.-butyrolactone + tetrahydrofuran system. Part 1: Densities of .gamma.-butyrolactone + water at 303.15-to 343.15 K and of tetrahydrofuran + .gamma.-butyrolactone at 278.15-to 298.15 K; and ultrasonic velocities at 298.15 K for the three binary systems inclusive of tetrahydrofuran + water and the ternary system tetrahydrofuran + water + .gamma.-butyrolactone