Calculate the shortest distance from a list of
points to all other pixels in an image, using the
Multistencil Fast Marching Method (MSFM). 

This method gives more accurate
distances by using second order derivatives and cross neighbours.

T=msfm2d(F, SourcePoints, UseSecond, UseCross)

# inputs,
  F: The speed image. The speed function must always be larger
			than zero (min value 1e-8), otherwise some regions will
			never be reached because the time will go to infinity.
  SourcePoints : A list of starting points [2 x N] (distance zero)
  UseSecond : Boolean Set to true if not only first but also second
               order derivatives are used (default)
  UseCross : Boolean Set to true if also cross neighbours
               are used (default)
# outputs
  T : Image with distance from SourcePoints to all pixels
shortestpath(distancemap, startpoint, sourcepoint, stepsize)

This function traces the shortest path from `startpoint` to
`sourcepoint` using Runge Kutta 4 in a 2D `distancemap`.

Optionally specify the line tracing `stepsize`.

Returns a 2D-array with the shortest path.