Fix literal rhythm application

[trevorbaca]

This works as expected:

maker(
    (['vn', 'va'], 1),
    baca.make_notes(),
    )

This mangles score output:

maker(
    (['vn', 'va'], 1),
    baca.rhythm("{ c'1 }"),
    )

Problem probably arises because rhythm is initialized only once; this will generate a reference problem causing violin and viola to point to same components. Change command to store input string and then evaluate input string once per score insertion.