Roots of spline

`x = fnzeros(spline)`

`x = fnzeros(spline,I)`

To find the roots of a spline, `fnzeros`

first converts the spline
to B-form. The function then performs some preprocessing to handle discontinuities and then
uses the algorithm of [1].

[1] Mørken, Knut, and Martin
Reimers. "An unconditionally convergent method for computing zeros of splines and
polynomials." *Mathematics of Computation* 76, no. 258
(2007): 845-865.