Cubic spline interpolation

Some tests, mainly related to derivatives, still fail.
Fixed broken cubic polynomial on coef 3
Fixed some typos in tests
Raised tolerance for hessian evaluation for cubic polynomials from cbrt(eps()) to sqrt(sqrt(eps()))
Avoid unnecessary allocation of temporary object
Documentation for InterpCubic
Merge pull request #19 from tlycken/cubic-spline-interpolation

Cubic spline interpolation