Remove unnecessary long/StatusCode conversions

In preparation for a larger re-design of StatusCode clean up a few places where the code unnecessarily relies on an implicit long type-conversion. In detail:

• ObjectVector::remove: Return the index of the deleted object as documented in the base class. Before it always returned 1
• RegistryEntry: Change return type of some methods from long to StatusCode
• IODataManager: Get rid of the S_OK/S_ERROR enum alias as this relies on the actual type of the StatusCode enums
• RootDataConnection: Change return type of setCompression from long to StatusCode
• Replace a few bool-casts with isSuccess() and long-casts with getCode()