Speed up cluster merge by batch copying arrays

[bouk]

I was looking through the code and found that a lot of time was being spent on 'make clusters' so I did a quick look through why that is the case. I found the merge_clusters function to be inefficient, so I optimized it to do batch memcopies wherever possible.

This gives me a ~20% reduction in time spent for a large image.

Before

After

[bouk]

@christian-rauch addressed your comments!