Support function returning IntervalBox and only use them internally

[Kolaru]

Currently, the functions passed to roots must return SVector which has several drawbacks:

• Require one more explicit import from the user.
• IntervalBox and SVector work in a quite similar manner, but are not equivalent, which may cause unexpected bugs. For example consider the following

IB = IntervalBox(emptyinterval(), emptyinterval())
SV = SVector(emptyinterval(), emptyinterval())

Here isempty(IB) evaluates to true while isempty(SV) evaluates to false. This caused some of the bugs I'm trying to fix in #93 (an update is coming soon).

Therefore, I advise that SVector should only be used when ForwardDiff is directly involved, i.e. deep inside the contractors.

It may be possible to support both the current behavior and function returning IntervalBox (converting to the latter internally) using fancy multiple dispatch, otherwise this would be a breaking change.

[dpsanders]

Good point and good idea.

For ForwardDiff I think we just need

import ForwardDiff

ForwardDiff.gradient(f, X::IntervalBox) = ForwardDiff.gradient(f, X.v)
ForwardDiff.jacobian(f, X::IntervalBox) = ForwardDiff.jacobian(f, X.v)
ForwardDiff.hessian(f, X::IntervalBox) = ForwardDiff.hessian(f, X.v)