Toumani Diabate, the great Malian kora player, rarely makes it to these shores, so his recent performance at Yoshis in Oakland was a stellar occasion. Known for his virtuosity, Diabate is a living master of the kora, the 21-string West African harp made of wood,metal, and a large calabash gourd. The kora has a characteristic sound, one that is hard to categorize. As he illustrated during the portion of the show when he talked to the audience about his instrument, he plays it with his thumbs a