double complex is non-standard

The `double complex` declaration used in obe.f90 is non-standard. For the code

```fortran
implicit none
double complex :: z
end
```

`gfortran -c -std=f2018 x.f90` gives

```
x.f90:2:15:

    2 | double complex :: z
      |               1
Error: GNU Extension: DOUBLE COMPLEX at (1)
```

Instead, `complex (kind=dp)` with dp a kind constant is standard.