disambiguate-add-year-suffix should only add suffix if year is the same

[erazlogo]

Right now the option adds suffixes for all entries for the same author, even if years of publication are different:

Thompson, E. (1995a). Machines, music, and the quest for fidelity: marketing the edison phonograph in america, 1877-1925, Musical Quarterly, 79(1), 131-171.

Thompson, E. (1999b). Listening to/for modernity: architectural acoustics and the development of modern spaces in america In P. L. Galison & E. Thompson (Eds.), The Architecture of Science (pp. 253-280). Cambridge, Mass.: MIT Press.

Thompson, E. (2002c). The soundscape of modernity: architectural acoustics and the culture of listening in america, 1900-1933. Cambridge, Mass.: MIT Press.

[simon]

(In [2432]) closes #954, disambiguate-add-year-suffix should only add suffix if year is the same