Multivalued function interpolation

Performs meaningful interpolation of multivalued functions

When interpolating a multivalued function (more than one y for each x), interp1 produces, at best, an error (if there are identical x data), and at worst, garbage (see the attached image). This function performs meaningful interpolation of a multivalued function, when the inverse (x = g(y)) is a well-defined function. For each interpolated point, interp1Multivalued returns the interpolated y existing on each branch defined in the input x and y vectors.