Handle vector types more gracefully

[Kolaru]

Currently no check is performed to ensure consistency of vector types, mainly Vector and SVector (see #207).

I think that the return type of the function should be checked and the input interval should be converted accordingly.
Namely for roots(f, X)

• f(X)::T, X::S -> convert X to T (typically T == SVector and X == Vector)

We could also check the jacobian, if it is given explicitly.