terms is a generic method
README.md updated
terms function removed, terms is a Generic method
[.CurveInterpolation always return a CurveInterpolation, even when it has a single element

When a CurveInterpolation has a single element it dispatches the method
call [.CurveInterpolation to [.SpotRateCurve
Curve interpolation handles only the 'at least two' error, others are thrown