Extract into new procedure `set_all_extr`

Extract definition of extrema from procedure `set_all_outerm` into new
procedure `set_all_extr`. So we are reverting part of commit
f5f4298d. The drawback, for now, is that `innermost_level` has to
transit in the main program unit from `set_all_extr` to
`set_all_outerm`.