Add metadata to spike recording

[thorstenhater]

Note: split off from #1929.

Add a metadata field to the spike callback, so that users can filter/attribute spikes.
Suggestions:

• location as given to place
• label as given to place

[thorstenhater]

After some exploration: There is just no way of handling this without adding a disastrous amount of extra storage.
This makes the decision to integrate detectors into the decor a bit awkward since the mapping (gid, idx) => label
must now be scavenged by the user from decor::placements or by design.

We might come back to this in the future, but for now: closing.