Rename to ```tau_vacant``` to ```decay_vacant```

[ackurth]

In the structural plasticity routine vacant (i.e. at a given time unused) synaptic elements decay with a "rate" (word used in the source code) tau_vacant, also referred to as a "time constant" in one case. I would argue the decay constant is neither a rate (units 1 / time unit) nor a time constant (units of time unit) , but a quantity without any unit. Since tau_vacant insinuates a time constant I suggest renaming the decay constant in just decay_vacant (in line with the naming for this in this paper referring to it as a decay constant). Also, in one case it is required that the decay constant is greater than 0. I find this impractical and unnecessary and thus suggest to alter this to greater or equal than 0.

[github-actions[bot]]

Pull request automatically marked stale!

[heplesser]

@ackurth Could you follow up the comments and merge master?