lagrange interpolation and derivative

This function performs the Lagrange interpolation of a function (y) or its derivative (dy/dx).

usage:
y=lagrange(x,pointx,pointsy,0) or
dy=lagrange(x,pointx,pointsy,1) or
dy=lagrange(x,pointx,pointsy,2)

There are two ways to find the derivative. If dydx_on (varargin(4)) is set to 1, it will base the derivative on pointx and pointsy (y will be dy/dx). If dydx_on == 2, the derivative is based on the interpolated points.

pointsy can be an array.

This function can get slow for large amounts of control points. Also, make sure to have extra control points outside the region of interpolation interest if there are lots of control points.