MNT: Revert spline coefficient initialisation boundary condition

This MR reverts the spline coefficient initialisation boundary condition for constant/zeros extrapolation to preserve the existing/established behaviour. The old behaviour for constant/zeros extrapolation was to use an initialisation method suitable for mirror extrapolation.

This is not strictly the best approach, which is why I changed it in !17 (merged). However, the old logic has been in place for at least 16 years, and so arguably the better option is to preserve the existing behaviour. We can revisit this in the future if needed.