Moving from vectors of pointers for nodes stored in the deactivated subdomain structs to vectors of node ids enables copy semantics.
For this the MeshLib now uses v3-ranges library and provides new
ids "function" (a view closure) (will be used in many other places; partly in this MR and in another upcoming MR(s)).
Consequently removing the unique pointers and storing values directly simplifies the constructors and many other parts of the code.
Fixes #3302 (closed)
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