Lost in all the hoopla surrounding the impending departure of Pittsburgh Symphony music director Mariss Jansons is the memory of how long it took the orchestra to adjust to its leader. Jansons’s style was so different from that of his predecessor, Loren Maazel, that it was several years before musicians and conductor seemed to feel at ease with each other. But the result of the collaboration has been widely deemed to be worth all the effort spent building a rapport – in recent years, the PSO has been hailed by critics as regularly coming up with “once-in-a-lifetime” performances. Jansons bows out this weekend, with the ultimate curtain-closer: Beethoven’s 9th.