* New binary arithmetic operators for PiecewiseLinearFuzzyNumbers: +,-

* piecewiseLinearApproximation() argument knot.alpha
  now defaults to equally distributed knots (via given knot.n)

* PiecewiseLinearFuzzyNumber() now accepts missing a1, a2, a3, a4,
  and knot.left, knot.right of length knot.n+2. Moreover, if \code{knot.n}
  is not given, then it is guessed from \code{length(knot.left)}.
  If \code{knot.alpha} is missing, then the knots will be equally distributed
  on the interval [0,1].