ENH: fitpack2: add _from_tck classmethod to BivariateSpline

similar to the _from_tck to UnivariateSpline
ENH: interpolate: add the feature of constant extrapolation to 1d spline.
TST: interpolate: tweak the extrap modes tests
TST: fitpack: remove the docstring from a test function
Merge pull request #3917 from ev-br/pr-3697-1

splines, constant extrapolation