Bug when using stochastic variable xi

[kjohnsen]

CLEOSim's adding coordinates appears to run into problems trying to re-create the xi variable (since I just copy-and-pasted code from Brian to create the x, y, and z variables)

Nathan's experience:

I caught another cleosim/Brian2 interaction bug while working on getting the Wilmes and Clopath simulation going. Brian2 uses the name "xi" as a special variable for white noise https://brian2.readthedocs.io/en/stable/user/equations.html. For some reason, in cleosim utilities, you redefine xi again, the same way Brian2 does. This creates the error: KeyError: "The name 'xi' is already present in the variables dictionary.". Adding a suffix (as Brian2 allows for) to the xi name, still gets the error again. (For instance I renamed xi to xi_model and got KeyError: "The name 'xi_model' is already present in the variables dictionary.")
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This bug was not showing up when I was just putting the brian2 network into a cleosim sim, but is now showing up that I am assigning coordinates using assign_coords_rand_rect_prism

Look at # Stochastic variables in utilities.py