Rename the "force" and "force_coods" attributes of Spline

[leouieda]

Granted these are not the best names and they are never used in the spline interpolation literature. They should be renamed to:

• nodes: the coordinates of the spline nodes (replaces force_coords).
• coef: the spline coefficients (replaces force)

These are more consistent with the literature. coef is also used in Trend so that makes sense.

[leouieda]

Must also be renamed in SplineCV.