Checks if the absolute path specified in the al_path
            is valid for current tree

def b_pathOK(self, al_path):
            """
            Checks if the absolute path specified in the al_path
            is valid for current tree
            """
            b_OK  = True
            try:          self.l_allPaths.index(al_path)
            except:       b_OK    = False
            return b_OK