Make queues for copying device variable once every x timesteps instead of every timestep

[esinyavuz]

As memory transfers between the CPU and the GPU are costly, it makes sense to queue some stuff in the memory and copy it once it is big enough. The same applies for writing results to files, etc.

[tnowotny]

We should probably use one spike buffer for 1) delays, 2) output buffering like here, 3) MUSIC integration buffering