make it possible to use IAF with from_model

[nkupelioglu]

• I cannot think of a better way of implementing.
• I cannot find a case where this would fail. Unit tests have been changed accordingly.
• A comment can be made on the root-cause of the issue is that SpikeLayer constructors do not share the same signature. I believe this was intentional and having the non-shared parameters in a dictionary covers all the cases, as long as the users do not use it in a wrong way. I also do not have a suggestion on how to address this without extensive discussion.
• This does not concern the Quality control.
• Changes are automatically reflected in the documentation.