Record communication stats per-subphase

[PhilMiller]

What Needs to be Done?

• Extend the instrumentation of objects' communication loads to record them at subphase granularity
• Update the CommOverhead load model to use this

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Additional context

[lifflander]

@bradybray Can you work on this next? We added subphases to ElementStats and NodeStats, but the communication information that is recorded is not stored per subphase (only per phase). Expand communication to be assigned a subphase, depending on which one is active.

[bradybray]

@lifflander Ok; I think I found where I'll be making the changes. I'll start here.

vt/src/vt/vrt/collection/balance/elm_stats.cc

Lines 184 to 196 in 14b814c

	CommMapType const&
	ElementStats::getComm(PhaseType const& phase) {
	comm_.resize(phase + 1);
	auto const& phase_comm = comm_[phase];
	vt_debug_print(
	lb, node,
	"ElementStats: getComm: comm size={}, phase={}\n",
	phase_comm.size(), phase
	);
	return phase_comm;
	}