Add init argument to set the core_atom_ids in BuildingBlocks that are often used for alignment.

[andrewtarzia]

These atom ids are often used to define alignment vectors, and users may want full control over them.

[lukasturcani]

Yeah agreed.

[andrewtarzia]

Further to this, an alternative may be something like the guest_start/target input for host_guest.Complex topology graph, which allows the user to actually define the vector for alignment as input.

I.e. the user may want to define some arbitrary building block A, which they want to react with a building block B and they may be able to define a vector (based on atom positions of A) which is used to define the vertex placement/edge direction between A and B?