Default date adapter

_Originally posted by @cwmrowe in https://github.com/akveo/nebular/issues/1056#issuecomment-565074447_

I have been having issues with the datepicker too but I think I undertstand it now.

Any time you import `NbDatepickerModule` and you want to use the format function you must include your format module (e.g. `NbMomentDateModule`) AFTER `NbDatepickerModule`. I believe this is because `NbDatepickerModule` provides `NbNativeDateService` (NatveDate) for use as the `NbDateService` by default, and in this case we want to override that with `NbMomentDateService` from `NbMomentDateModule`.

I think it would be simpler if Nebular made you choose the default `NbDateService`, rather than making `NbNativeDateService` default and forcing you to override it every time you include `NbDatepickerModule`.