Simplify 'computeDurationTensions'

Moves the addition of interval tensions and duration tensions from the
`computeDurationTensions` function to `runTheSPEACWeightings`.
Fix bugs in 'derive', 'findMotionWeightings', 'getDurations'

* https://github.com/sjakobi/i-love-emily/commit/0cc746877d9eebbfa2bd8ddcb03b6620e19f3569#commitcomment-7502433

* https://github.com/sjakobi/i-love-emily/commit/2f409344fd5e90696be2fe097027fd6a24f68f2e#commitcomment-7502439

* Replace 'tail' by 'drop 1' in 'getDurations' in analogy to the fix for
  'findMotionWeightings'.