Get start of string to center it in an area
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 Syntax

       nPos := GT_Cent(<cStr>, <nLen>)

 Arguments:

     cStr := String to be centered
           nLen := Length of area to center string in

 Returns:

     Starting postion

 Description:

     To ascertain the the starting position of a string to
     enable it to be centered in a given area

 Examples:

           nPos := GT_Cent( space(20), 40 )
     nPos will equal 10

 Source: GT_CENT.PRG

 Author:  Niall Scott