Change root note handling

[tomcontileslie]

Currently the first element of the list of interpreted notes is considered the root, and the following notes are considered the important ones. I suggest we make the interpretation function output a tuple (out, root) where root is the bass note and out contains all the notes, still most to least important. It might be time to retire the rank_important function, in the sense that some important notes (root, fifth) are best places on the low strings while the more musical, but also important, ones (sevenths, ninths, etc) are better on high strings.