No orientation guard anywhere

Following from #454 and #453 : there is no `abs(J)`, no `sign(J)`, no assertion and no
warning about negative or vanishing Jacobians anywhere in `src/octave` or
`src/cpp`. Carrying the signed Jacobian is the right design; not being able to
find out that you are on the wrong side of it is the problem.

A one-line check in `jacobian2D` / `jacobian3D` — warn if `any(J < 0)` or if
`min(J)` and `max(J)` differ in sign — would have surfaced 2 and 3 immediately
and costs nothing.