Inaccurate delta stats of aggregated records

[zyiou]

Is your feature request related to a problem? Please describe.
Currently when aggregating records, we added delta stats https://github.com/vmware/go-ipfix/blob/main/pkg/intermediate/aggregate.go#L522-L528. Additionally, throughput calculation will make more sense when using accurate octetDeltaCount instead of source or destination specific delta stats.

Describe the solution you'd like
Assign maximum of the delta stats instead of summing up.

Describe alternatives you've considered
N/A

Additional context
N/A