Simplify 'removeOctaves'
Improve approach tension computations

* getChordRoots' is replaced by "map getChordRoot", where getChordRoot
  is a far simpler function than its predecessor (but quite different).

* rootStrengthAndRoot has been broken up into two functions,
  "intervalStrength" and "rootNote".
Update "interval", introduce (.:) operator
Remove unused collectSimultaneousEvents and swap functions
Rename "rootNote" to "intervalRoot" for consistency
Reorder code