Simplify memory profile

[brendanheywood]

Some profiles have a huge number of memory data points in the graph:

Thinking in a similar way to how we consolidate flamelets we could also reduce the amount of stored info here, both in memory as it is being collected and when persisted.

• First round it to some good enough precision. Probably a significant figures is better than a fixed precision
• If we have multiple adjacent memory items which are the same then just drop the later ones as they are redundant