FUNCTION line2d, x1, y1, x2, y2, NO_PAR_CHECK = no_par_check

; Description: This function calculates the y-intercept and gradient of a straight line passing
;              through the points A and B with coordinates "(x1,y1)" and "(x2,y2)", respectively.
;              The equation of the straight line used is:
;
;              y - y1 = ((y2 - y1)/(x2 - x1)) * (x - x1)
;
;              This equation is then rearranged into the form:
;
;              y = m*x + c
;
;              and the y-intercept c and gradient m are returned to the user as a 2-element
;              vector.
; 
; Input Parameters:
;   
;   x1 - FLOAT/DOUBLE - The x coordinate of the first point.
;   y1 - FLOAT/DOUBLE - The y coordinate of the first point.
;   x2 - FLOAT/DOUBLE - The x coordinate of the second point.
;   y2 - FLOAT/DOUBLE - The y coordinate of the second point.
;
; Return Value:
;
;   The function returns a two element vector of DOUBLES where the first and second elements are 
;   the y-intercept and gradient of the line, respectively. If the input parameters are not numbers
;   of the correct type (and parameter checking is performed), then both the y-intercept and 
;   gradient are returned as zeros. In the case that the straight line joining A and B is "vertical" 
;   (x1 = x2), then both the y-intercept and gradient are returned as zeros. In the case that A and 
;   B are coincident, then again both the y-intercept and gradient are returned as zeros.
;
; Keywords:
;
;   If the keyword NO_PAR_CHECK is set (as "/NO_PAR_CHECK"), then the program will not perform 
;   parameter checking on the input parameters, reducing program overheads.
;
; Author: Dan Bramich (dmb@ing.iac.es)
;
; History:
;
;   09/08/2008 - Module created (dmb)  

;Perform parameter checking if not instructed otherwise

  ;Check that "x1", "y1", "x2" and "y2" are all numbers of the correct type

;If the two points A and B are not coincident, or do not form a "vertical" straight line

  ;Calculate the line gradient

  ;Calculate the line y-intercept

  ;Return the straight line y-intercept and gradient as a two-element vector

;If the two points A and B are coincident, or form a "vertical" straight line, then return the 
;line y-intercept and gradient as zeros