Provide a "deep copy" operation on ISO 19115 metadata

In Apache SIS, all implementation classes of ISO 19115 metadata types have a shallow copy constructor. However there is nothing for performing a deep copy (the clone() method is not for this purpose). While deep copy should be avoided, it is sometime useful for example when using an existing metadata as a template.

Better alternative

Note that the ModifiableMetadata.unmodifiable() method provides a better way to use a metadata as a template, as it returns a snapshot and allows the caller to continue to modify the original metadata object and create new snapshots. This approach allows sharing the children that have the same content, thus reducing memory usage. In comparison, deep copy operations unconditionally duplicate everything, no matter if it was needed or not. Nevertheless deep copies are still sometime useful, for example when we do not have the original ModifiableMetadata instance anymore.