Disable dataset end-points in core that require domain specialisation

[csterling]

E.g. the merge end-point requires knowledge of how to merge the annotations, so the core end-point should be disallowed.

[csterling]

Instead of disabling the core methods where domain-specific functionality is required, data-sets now automatically up-cast to their domain-specific variants when called from a core method, so the functionality is consistent with the domain-specific methods.

Implemented in e8c530d1b86e9a400999d29125f795f6474189fb.