With the ''Concerto for Orchestra,'' Henle inaugurates a series of affordable study scores based on the text of the Bartók Complete Edition. When Bartók received the commission to compose this piece fromSerge Koussevitsky in May 1943, it meant not only financial support, but also an important creative impetus for the gravely ill composer. By October, the five-movement concerto, which Koussevitsky enthusiastically dubbed ''the bestorchestral work of the last 25 years,''