review ad-hoc repair of DiffUtil::meanRelVecDiff; investigate tiny numbers in examples/scatter2d/Resolution

to get Resolution.py under test coverage, I added

	if (ref[i] == 0 && dat[i] == 0)
	    continue;
	if (ref[i] == 0 || dat[i] == 0) {
	    std::cerr << "check manually at i=" << i << ": ref=" << ref[i] << " vs dat=" << dat[i] << "\n";
	    continue;
	}

to DiffUtil::meanRelVecDiff.

Test log shows

check manually at i=76: ref=0 vs dat=3.20155e-320
check manually at i=49: ref=0 vs dat=4.00193e-322
check manually at i=50: ref=0 vs dat=4.00193e-322
check manually at i=51: ref=0 vs dat=4.00193e-322
...