Reflecting BC for quadratic itp

[tomasaschan]

I realized that for quadratic interpolation, Flat (first derivative equal to zero
at edges) and Reflect (all continuous derivatives equal to zero at edges) are equivalent,
since the first derivative is the only continuous one for quadratic interpolation.
This just adds the Reflect option so you can do

itp = Interpolation(A, Quadratic(Reflect(), OnCell()), ExtrapReflect())

which feels a little more intuitive than using Flat even though they're the same.