fix subphases handling in getModeledComm

[cz4rs]

Describe the bug
Currently RawData::getModeledComm does not consider subphases (note: this method might get moved out of RawData model). Fix that by handling them the same way CommOverhead does.

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vt/src/vt/vrt/collection/balance/model/comm_overhead.cc

Lines 78 to 84 in 3e362c4

	if (offset.subphase == PhaseOffset::WHOLE_PHASE) {
	return work + overhead;
	} else {
	// @todo: we don't record comm costs for each subphase---split it proportionally
	auto whole_phase_work = ComposedModel::getWork(object, PhaseOffset{offset.phases, PhaseOffset::WHOLE_PHASE});
	return work + overhead * ( static_cast<double>(work)/whole_phase_work );
	}

(notice the old todo here as well)

Originally posted by @cz4rs in #1695 (comment)