Clean up
Interpolation functions have been moved to Interpolation.R
SpotRateCurve constructor receives the interpolation method's label, instead of the interpolation function
SpotRateCurve's interp method created

This method interpolates based on interp attribute defined to the curve
interp.FlatForward isn't a method, anymore.
4 interpolation methods implemented

. Linear
. Natural Cubic Spline
. Hermite Cubic Spline
. Monotone Preserving Cubic Spline