Object-oriented interface for 1D and 2D splines

New object-oriented wrappers:

• fitpack_curve: 1D Spline functions from scattered data (smoothed/interpolating/least-squares)
• fitpack_periodic_curve: periodic 1D Spline functions
• fitpack_parametric_curve: parametric curve in N dimensions
• fitpack_closed_curve: closed parametric curve in N dimensions
• fitpack_constrained_curve parametric N-D curve with enddpoint constraints (value and 1:k derivatives)
• fitpack_surface: 2D Spline functions from scattered (x,y) data in cartesian coordinates
• fitpack_polar: 2D Spline functions from scattered data in polar coordinates (u in [0,1], v in [-pi,pi]), and user-provided boundary of the polar domain (circle, ellipsoid, etc.)
• fitpack_sphere: 2D bicubic Spline function from scattered data on a spherical domain (Latitude or theta in [0,pi], Longitude or phi in [0,2pi])

New property wrappers:

• dfdx = derivative(s)
• integral = integral
• roots = roots of a cubic spline
• fourier_coefficients = extraction of the Fourier coefficients for a user-defined list of harmonics

Full Changelog: 0.1.0...0.2.0