add external (existing) template files and instrution files to the
        Pst instance

def add_external(self):
        """ add external (existing) template files and instrution files to the
        Pst instance

        """
        if self.external_tpl_in_pairs is not None:
            if not isinstance(self.external_tpl_in_pairs,list):
                external_tpl_in_pairs = [self.external_tpl_in_pairs]
            for tpl_file,in_file in self.external_tpl_in_pairs:
                if not os.path.exists(tpl_file):
                    self.logger.lraise("couldn't find external tpl file:{0}".\
                                       format(tpl_file))
                self.logger.statement("external tpl:{0}".format(tpl_file))
                shutil.copy2(tpl_file,os.path.join(self.m.model_ws,
                                                   os.path.split(tpl_file)[-1]))
                if os.path.exists(in_file):
                    shutil.copy2(in_file,os.path.join(self.m.model_ws,
                                                   os.path.split(in_file)[-1]))

        if self.external_ins_out_pairs is not None:
            if not isinstance(self.external_ins_out_pairs,list):
                external_ins_out_pairs = [self.external_ins_out_pairs]
            for ins_file,out_file in self.external_ins_out_pairs:
                if not os.path.exists(ins_file):
                    self.logger.lraise("couldn't find external ins file:{0}".\
                                       format(ins_file))
                self.logger.statement("external ins:{0}".format(ins_file))
                shutil.copy2(ins_file,os.path.join(self.m.model_ws,
                                                   os.path.split(ins_file)[-1]))
                if os.path.exists(out_file):
                    shutil.copy2(out_file,os.path.join(self.m.model_ws,
                                                   os.path.split(out_file)[-1]))
                    self.logger.warn("obs listed in {0} will have values listed in {1}"
                                     .format(ins_file,out_file))
                else:
                    self.logger.warn("obs listed in {0} will have generic values")