Change coords1d ddt metadata units to Pa

During review of #370, @peverwhee wrote:
> Is Pa correct? The metadata says that this has units = none

about
```
type(coords1d), intent(in) :: p ! Pressure coordinates for solver [Pa]
```

@jimmielin wrote:
> It's a ddt with the underlying units as `Pa`. I wasn't sure why I noted `units = none` in the metadata (it's what was done for HB), maybe I set the ddt units as `none` out of habit, but the comment is more correct.

I think if it works to annotate `units = Pa` for the ddt it should be that way.