introduced MutableAsset for discovered assets (see details)

- MutableAsset extends Asset to allow Repository to set a repository
service on assets discovered by Browsers. This simplifies service plugin
design, as Broswers no longer have to be aware of their services.
- AbstractAsset now implements MutableAsset
- Asset class constructors no longer expect a service, except when
convenient (e.g. SdmxAsset). In other cases, clients are responsible for
setting services before publication.
- Repository now checks for services upon retrieval and publication, as
constructors no longer ensure their existence