Document GPUSpikingCluster code used only for validation

[stiber]

What kind of issue is this?

• Bug report
• Feature request
• Question not answered in documentation
• Cleanup needed

And explain why.

What is affected by this?

simulator

How do we replicate the issue/how would it work?

Expected behavior (i.e. solution or outline of what it would look like)

Other Comments

[stiber]

So, for example, GPUSpikingCluster::GenRandNumbers() is used only for validation, generating random numbers in cluster 0 for distribution to other clusters. Search in that file for "VALIDATION".