Class find_embedding::fixed_handler_list

class find_embeddingfixed_handler_list

This fixed handler is used when variables are allowed to be fixed after instantiation.

For that functionality, we probably need…

• dynamic modification of var_neighbors and qubit_neighbors to maintain speed gains: fixed variables are sinks, reserved qubits are sources.
• access to / ownership of var_neighbors and qubit_neighbors in this data structure
• move existing initialization code from find_embedding.hpp into fixed_handler_hival (note the interplay with shuffling qubit labels, this might get gross)