Simplify 'computeDurationTensions'

The addition of interval tensions in `computeDurationTensions` has been
removed. To make up for this, and in order to counterbalance rounding
errors in the computation of the SPEAC weightings for the bookExample,
the interval tensions are multiplied with a factor of 1.11 during
computation in `createListOfTensions`.
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'.