ENH: adding constant extrapolation from boundary to 1d spline, _from_tck function to 2d spline

This is a rebase of the PR #3339, reflecting the reviews by Andreas and Evgeni.

The remaining issues are the unittests for the functions _from_tck of UnivariateSpline and BivariateSpline.

https://github.com/scipy/scipy/pull/3339