Change setups

[damiansteiger]

Remove setup parameter from MainEngine (has not yet been merged to Master).

This parameter has been introduced to make the compiler engine specifications more explicit instead of just import projectq.setups.ibm. Now it is even more explicit: One needs to set the engine_list parameter:

MainEngine(backend=..., engine_list=projectq.setups.ibm.get_engine_list())

Advantages is that one can give the function get_engine_list parameters, e.g., we can build a compiler engine setup for a linear chain and give it as a parameter the length of the chain:

import projectq.setups.linearchain as linearchain
MainEngine(backend=..., engine_list=linearchain.get_engine_list(num_qubits=8))

Downsides:

• It takes a bit more of writing but is much more flexible
• The simplicity of the setup parameter was nice, maybe in future we can use that parameter to set the engine_list and change the default backend in case the user does not provide one (Previously it only set the engine_list)

Other options to this PR:

• add setup_parameters which should take a dict in order that we can check correct parameter names as each setup might have different ones.
  I.e.

MainEngine(backend=..., setup=projectq.setups.linearchain, setup_params={num_qubits: 8})

[thomashaener]

Thanks! 👍