Apply Rule of Zero across utility/ helpers (#692)
## Summary
Remove user-declared destructors and trivial copy ctor/assignment
implementations whose bodies are equivalent to the compiler-generated
defaults. The implicit special members are correct in every case (no
owning raw pointers, no side effects in the removed bodies, virtual
destructor inheritance preserved through base classes).
### utility/graph/ — non-owning iterator/element helpers
* `Graph.hh`: `EdgeListElement`, `EdgeListIterator`,
`EdgeListConstIterator` — drop user dtor and the `= default` copy ctor /
assignment.
* `Digraph.hh`: `DirectedEdgeListElement`, `DirectedEdgeListIterator`,
`DirectedEdgeListConstIterator` — drop user dtor and member-wise copy
ctor / assignment.
* `ArrayPool.hh` (`Array0`): drop user dtor, copy ctor, and
self-assignment-guarded copy assignment.
* `LowMemGraph.hh` (`LowMemGraphBase`): drop empty `override` dtor
(`utility::VirtualBase` supplies the virtual destructor).
* `UpperEdgeGraph.hh`: drop the empty `UEEdge` dtor and
`~UpperEdgeGraph() override = default;`.
`LowMemNode` / `LowMemEdge` are intentionally left untouched — their
explicit destructor declarations anchor a load-bearing comment about
deliberately not making the destructor virtual.
### utility/
* `DereferenceIterator.hh` (`DereferenceIterator`): drop empty user
dtor.
### utility/options/ — empty overriding destructors
Drop `~XxxOption() override {}` from the eight leaf scalar/vector option
classes. The implicit destructor is virtual via inheritance from
`Option` (which declares a virtual destructor), so dynamic dispatch is
preserved.
* `BooleanOption.hh`, `BooleanVectorOption.hh`
* `FileOption.hh`, `FileVectorOption.hh`
* `IntegerOption.hh`, `IntegerVectorOption.hh`
* `RealOption.hh`, `RealVectorOption.hh`
The remaining option classes with the same pattern (`PathOption`,
`StringOption`, `ResidueChainVectorOption`, `ScalarOption`,
`ScalarOption_T_`, `VectorOption`, `VectorOption_T_`) can be addressed
in a follow-up; the eight here form a coherent leaf-scalar-plus-vector
subset.