Put getDocument logic in HydrationSettingsInterface

Do no longer require objects that use custom HydrationSettings to
implement Indexable.
Fix E_ERROR

PHP Fatal error:  Declaration of
vierbergenlars\Norch\ODM\HydrationSettings\SingleObjectHydration::getDocument()
must be compatible with
vierbergenlars\Norch\ODM\HydrationSettingsInterface::getDocument($document)
in /home/lars/Documents/GitHub/Norch-
PHP-Client/lib/vierbergenlars/Norch/ODM/HydrationSettings/SingleObjectHydration.php
on line 12