Fix free boundary condition on Cubic

[maltezfaria]

• Fixes a bug on the Free boundary condition for Cubic interpolation, which was previously just a copy-past of the periodic boundary conditions.
• Adds some tests that cubic interpolation should be exact for cubic polynomials when using the free boundary condition.

Should close #594 (and possibly #570?).

[maltezfaria]

The CI failure seems to be unrelated to the changes in this PR. Not sure how to go about it though (change the broken tests in master to @test_broken?).

[maltezfaria]

And here is an visual example of the effect of the BC when running this code:

using Interpolations, CairoMakie
ix = 1:10
f = x -> x^3 - 2x^2 - 1
A = map(f, ix)
itp = interpolate(A, BSpline(Cubic(Free(OnGrid()))))
xx = 1:0.01:10 |> collect
fig, ax, pl = lines(xx, f; color = :black, label = "original", linestyle = :solid, linewidth = 2)
lines!(ax, xx, map(itp, xx); color = :blue, label = "interpolated", linestyle = :dash, linewidth = 2)
axislegend(ax, position = :lt)

master

PR

[mkitti]

Watching ...

[ajwheeler]

CI failures should be fixed after #614 and #615 are merged. Confirmed that the tests failing here are the same ones failing on master.

[mkitti]

I just merged the above two pull requests. I'll come back to this after those changes settle in. It does not appear there are any conflicts between this PR and what I just merged.

[maltezfaria]

@mkitti thanks! Let me know if you need a rebase at some point.

[mkitti]

Thank you. Sorry for the delay.

[maltezfaria]

Thank you. Sorry for the delay.

Thanks!