PRO triangles_merge, tr1, tr2, ntr1

; Description: This module merges the triangles specified in the array "tr1" with the triangles
;              specified in the array "tr2". The final triangle list is returned via the array
;              "tr1", and contains only unique triangles that have no repeated points as vertices.
;              The final triangle list also has the vertex indices sorted in ascending order for
;              each triangle. The number of triangles in the final triangle list is returned via 
;              "ntr1".
;                By deliberately setting the second triangle list "tr2" to "0" (or even an empty
;              variable), then this module will simply return "tr1" with only unique triangles
;              that have no repeated points as vertices, and that have the vertex indices sorted
;              in ascending order for each triangle.
;
; Input Parameters:
;
;   tr1 - INTEGER/LONG ARRAY - The first triangle list of Nt1 triangles stored as a 3 by Nt1 array
;                              of numbers where each triplet of numbers represents the indices of
;                              the points corresponding to the three vertices of each triangle.
;   tr2 - INTEGER/LONG ARRAY - The second triangle list of Nt2 triangles stored as a 3 by Nt2 array
;                              of numbers where each triplet of numbers represents the indices of
;                              the points corresponding to the three vertices of each triangle. 
;
; Output Parameters:
;
;   tr1 - LONG ARRAY - The final list of "ntr1" unique triangles stored as a 3 by "ntr1" array of
;                      LONGS where each triplet of numbers represents the indices of the points
;                      corresponding to the three vertices of each triangle. This list of triangles
;                      has no triangles with repeated points as vertices, and the vertex indices
;                      for each triangle are sorted in ascending order.
;   ntr1 - LONG - The number of triangles stored in the final list of triangles "tr1".
;
; Author: Dan Bramich (dmb@ing.iac.es)
;
; History:
;
;   03/08/2008 - Module created (dmb)

;Test the dimensions of the triangle lists, and return the relevant output if both input triangle 
;lists are either empty or not of the correct number type or not of the correct dimensions

;If the first triangle list is either empty or not of the correct number type or not of the correct dimensions, then replace the
;first triangle list with the second triangle list

;If both triangle lists are non-empty and of the correct dimensions, then append the second
;triangle list to the first triangle list

;Remove triangles from the output triangle list that have repeated points as vertices

;For each triangle in the output triangle list, sort the vertex indices in to ascending order,
;and calculate a unique identifying quantity

;Reduce the output triangle list to a set of unique triangles