tvm 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