title: Method to find x,y point on a spline that is an exact distance from another point
body: <p>I have a 2-d Spline that is a list of (x,y) tuples. I need to determine a point on the spline where a line of length L falls if one end of the line is placed on an existing spline point.</p>
  
  <pre><code>spline=[(x1,y1),(x2,y2),(xn,yn).....]
  Line=200
  for i in spline:
      find (xn,yn) 200 units from here
  </code></pre>
  
  <p>So for each point in the spline, find a (Xn,Yn) (or interpolated value) that is a linear distance of 200 from the chosen point.</p>