Modernize copy prevention in core factory singletons and non-copyable classes (#685)
- **SilentStructFactory, ConstraintFactory, SequenceFactory**: all
inherit from `utility::SingletonBase`, which already `= delete`s copy
construction and copy assignment. Removes the now-redundant
private-but-unimplemented declarations from each derived class
(pre-C++11 idiom).
- **OrbitalsStatistics, AtomTreeDiff**: had private-but-undefined copy
constructors (pre-C++11 idiom); replaced with explicit `= delete` on
both copy constructor and copy assignment operator.
No behaviour change; build-only verification is sufficient for pure
copy-prevention modernisation.