merge some read_nbf/nif functions

> grep '^subroutine read_nbf' *f90 | grep fch                                                                    
py2fch.f90:subroutine read_nbf_from_fch(fchname, nbf)
rwwfn.f90:subroutine read_nbf_and_nif_from_fch(fchname, nbf, nif)
> grep '^subroutine read_nif' *f90                                                                     
file_op.f90:subroutine read_nif_from_fch(fchname, nif)

Probably we don't need read_nbf_from_fch and read_nif_from_fch, they can be replaced by read_nbf_and_nif_from_fch.