Subroutine:	MATVEC_PRODUCT

 Purpose:      3-dimensional matrix - vector product                   

 Category:	MATH

 File:		irco.shl

 Author:	Do Kester

 Use:	call MATVEC_PRODUCT( 
		RMAT, 	I	double precision array(3,3)
		X, 	I	double precision array(3,N)
		Y, 	O	double precision array(3,N)
		N )	I	integer
       N     Number of X and Y vectors                     
       X     an array with N 3-dimensional (column) vectors
       RMAT  a 3 x 3 matrix                                
       Y     an array with N (column) vectors, Y = RMAT times X
                                                                       
 Description:
       Mainly meant as a shorthand for                                 
         Y(i,j) = RMAT(i,1) * X(1,j)                                   
                + RMAT(i,2) * X(2,j)                                   
                + RMAT(i,3) * X(3,j)                                   
                                                                       
       Use it with N=1 for simple 3-dimensional vectors,               
              with N=3 for 3 x 3 matrix multiplication,                
              with arbitrary N for e.g.                                
                   large volume coordinate transformations.            
                                                                       
       It is allowed that X and Y are the same array.                  
                                                                       
       Peculiar things will happen if you use it for                   
       matrix multiplication, and RMAT and Y are the same array.       
                                                                      
 Update:       880112 Timo Prusti & Peter Arendz : standarized         
               870518 Albrecht de Jonge documentation improved         
               850625 Albrecht de Jonge