Changes in version 0.4                         

  - Add year fraction and compounding arguments to both xnpv and xirr,
    providing greater flexibility

                         Changes in version 0.3                         

  - Add vignette and NEWS
  - Prevent negative discount factors when calculating them from swap
    curves
  - Split zero rates in zero nominal and zero effective
  - A functor argument is added to the rate_curve constructor, to allow
    the user to specify how the interpolation should be performed.
  - Changing the default spline interpolation method from natural to
    monoH.FC, as to respect monotonicity if present
  - Add new irregular functions, xnpv and xirr