Feature/monotone cubic spline
Fixes #712 (closed).
Implements monotone cubic spline due to Fritsch & Butland (1984) (https://doi.org/10.1137/0905021)
Fixes #712 (closed).
Implements monotone cubic spline due to Fritsch & Butland (1984) (https://doi.org/10.1137/0905021)