rings const part_chords

[eh2k]

Two small improvements here.
The first is the const declaration of the chords array in the part.cc.
The second change (renaming chords to part_chords) is because of the case if you want to use the chords array with extern const float chords[] elsewhere. Imho it is not possible because there are two global chords definitions in the rings namespace. See one in the part.cc the other in the synth_string_part.cc