Simplified simulating only a subset of neurons.

Previously in order to do that, the range of neurons had to be a continuous range of indices in the YAML file. Now, any subset can be used.
Moved function to save spikes to meta routines.
Minor code simplifications.

No actual changes.
Merge commit '37286c8f7c88f120b7c2a94de65142f2498f91a6'