Draft: 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.