Use unordered map for indices

[annawendler]

Use unorderep map for indices to make access more clear, is needed for issue #378

[lenaploetzke]

We could possibly solve this similarly to the struct Flow in the file memilio/utils/flow.h (or use the struct). To do this, we would have to implement the get_flat_flow_index() function from the FlowModel again or move the function to a different location.