improved stat tracking / introspection (Imported #55)

[nmcglo]

Original Issue Author: Jonathan Jenkins
Original Issue ID: 55
Original Issue URL: https://xgitlab.cels.anl.gov/codes/codes/issues/55

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Applies to codes models and especially modelnet. Numerous contexts in which we want to know things like link utilization, contention, etc.

Additionally, need to decide whether we code this into the network LPs directly, or provide an event-based interface so that other LPs can get at the stats - especially useful for people doing e.g. dynamic torus routing.

[nmcglo]

Jonathan Jenkins:

modelnet now has a sampling interface, which models can opt-in implement. It would still be nice to have a generic way to "inject" such functionality, but I don't see a good way apart from drastic moves such as hijacking every LP.

[nmcglo]

For now stats are done on a per LP basis. They do need to be worked on, however. See #182