Allow for spline-matching continuity in extrapolation-range

[mgrub]

Is your feature request related to a problem? Please describe.
The spline interpolation at its current state has "edges" at the boundaries, if extrapolation becomes necessary. This is in line with the docstring, but not necessarily wanted.

Describe the solution you'd like
The spline should approach the values provided in fill_value and fill_unc without "edges". E.g. a cubic-spline should do so with C2-continuity. This could be achieved by padding the interpolation range with a suitable number of values from fill_value.