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.

[heplesser]

The documentation of tau_vacant states

 *  tau_vacant       double  - Rate at which vacant synaptic elements will decay.
 *                             Typical is 0.1 which represents a loss of 10% of the vacant synaptic
 *                              elements each time  the structural_plasticity_update_interval is
 *                             reached by the simulation time.

It think it definitely a rate, if in the time unit of structural_plasticity_update_interval. I think we should discuss more thoroughly how to define this quantity properly.

[clinssen]

I would suggest to keep it as a "fraction" or "proportion" and document that the multiplication is done every structural_plasticity_update_interval. Perhaps "reached by the simulation time" can be rephrased.

[github-actions]

Pull request automatically marked stale!

[heplesser]

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

[ackurth]

I close the PR, issue can be addressed when improving on structural plasticity in general.