Changes made in order to generate the curve interpolators approxfun and splinefun
LogLinear interpolation implemented
Solve

Using R's interpolation function, approx, with constant values for x
and y raises an exception.

approxfun(x=rep(1,10), y=rep(1,10))

This code raises the following exception

Erro em approxfun(x = rep(1, 10), y = rep(1, 10)) :
  são precisos ao menos dois valores não-NA para interpolar

Which obviously is correct because such a curve doesn't exist.
At least x should vary, like (1:10), for example.
SpotRateCurve now has a dib
Implemented, correctly, SpotRateCurve's forward.rate