The chord reducer now cleanly reduces BWV846 via arpeggio collapsing.
Improved chord reducer preprocessing algorithms.

The chord reducer now does a better job collapsing adjacent chords with
identical pitch sets.

Next we'll try and tackle pruning neighbor and passing tones.
Merge branch 'master' of https://github.com/cuthbertLab/music21
Identical chord collapsing is now a boolean option for the chord reducer.
Improved verticality analysis in chord reducer.
Improved verticality analysis in chord reducer.
Fixed augmented 6th chords in major.
Added missing entries for Vibraphone to instrumentLookup (thanks Chris
Culy)
Find instances of eval in code and remove them except for two in sieve
which are very hard and SEEM to be safe; notes in webapps about two
important uses of eval that have safety mechanisms but should later be
rewritten.  Remove dangerous eval in Instrument. Remove
obsolete/translation because of eval possibilities (keep in Git archive)
Implemented basic passing-tone removal in chord reducer.
Implementing offset mapping for chord reducer.

Part of the goal is to support comparison of vertical moments across measure
boundaries.
Implementing searchable offset-tree data structure.

The OffsetTree will support rapid lookups of simultaneous score elements.
Implemented search methods on OffsetTree.
OffsetTree now maintains parentage information of its elements.

This means that given an input score, the offset tree will keep track of the
entire containment chain in that score down to each note and chord. Keeping
track of parentage helps us group elements by voice or part across measure
boundaries.
Improved parentage-handling in OffsetTree.
Added coordinating information to offsetTree.Parentage.

Parentage objects now know the measure number, part name, start offset and stop
offset of their element.

They can also emit new parentages with different start and/or stop offsets via
a new() method. This will be useful once we implement mutators on the
OffsetTree, as we will be able to reposition elements within the tree while
maintaing their parentage information.
Cleaned up names in offsetTree.
Added an AVL tree implementation.

This will serve as the basis for a reimplementation of OffsetTree, supporting
both insertion and deletion.
Reimplemented AVLTree to work with timespans.
Improved AVLTree handling of timespans.