RefPort's sometimes handled a time step late

[PhilippPlank]

Issue Number: #204

Objective of pull request: Deterministic behavior of when data sent via a RefPort is received by the target process. Occasionally it was possible that the receiving process moved to the next phase before all of its VarPorts got serviced, because sending data via a RefPort does not block. Introducing of a wait() method for RefPorts makes sure that sent data is serviced before the process finishes (which blocks advancing a time step and ensures all VarPorts on the receiving process will be serviced beforehand).

Pull request checklist

Your PR fulfills the following requirements:

• Issue created that explains the change and why it's needed
• Tests are part of the PR (for bug fixes / features)
• Docs reviewed and added / updated if needed (for bug fixes / features)
• PR conforms to Coding Conventions
• PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• Lint (pyb) passes locally
• Build tests (pyb -E unit) or (python -m unittest) passes locally

Pull request type

Please check your PR type:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation changes
• Other (please describe):

What is the current behavior?

• Occasionally it was possible that the receiving process moved to the next phase before all of its VarPorts got serviced, because sending data via a RefPort does not block.

What is the new behavior?

• Introducing of a wait() method for RefPorts makes sure that sent data is serviced before the process finishes (which blocks advancing a time step and ensures all VarPorts on the receiving process will be serviced beforehand).

Does this introduce a breaking change?

• Yes
• No

Supplemental information

[awintel]

Looks fine. But you should update the docstring.

[bamsumit]

Tested it out on PN SNN notebook. The the results are consistent.

[awintel]

I would still recommend to improve the docstring.
The fundamental behavior of the RefPort should be explained in the class docstring.

Approving anyways since this is just a style issue that can be fixed later. Just don't forget about it.