interpolateXY performs piecewise linear interpolation of a single point, or a series of points onto a given function in the x-y plane.
Parameters:
- the x-y function:
- scalar field xOld; and
- <Type> field yOld.
- the interpolation point(s):
- for a single point, scalar xNew;
- for multiple points, scalar field xNew.
Returns yNew, a <Type> field.
