- Returns the minumum distance from the cell to the edge of the grid
   (variable name is result.LMIN in the IDL structure.

 - Output variable name can be given after the input structure in the
   IDL call. The resulting structure will be give as the value of the
   specifed variable (doesn't have to be defined before).

The Escape Probability RT length scale can be determined two ways:

   1) taking the minumum distance between the cell and the grid wall

   2) taking the average Av and devide it with the cloud average density

  (3) Calculate the mean density along each rays. This is complicated,
      this it was rejected.