Chi2Test: Report residuals also for higher dimensions

[phi-mah]

I wanted to perform a chi^2 comparison between two TH2 histograms (h1 and h2), making use of the TH1::Chi2Test method. I hoped to get access to the individual residuals as follows:

TH2D h_res {*h1};
h_res.Reset();

h1->Chi2Test(h2, "OF UF" , h_res.GetArray());

//h_res should now contain the residuals

It really took me some time to understand why this is not working. It turns out this is a bug(?) in the code. The expression that fills the residuals only considers one dimension (x, lookp variable i) and simply ignores y (j) and z (k):

root/hist/hist/src/TH1.cxx

Line 2256 in 9c4a8d2

if (res) res[i - i_start] = (cnt1 - nexp1) / TMath::Sqrt(nexp1);

I understand that a higher dimensional res array layout is more complicated, especially if custom limits are involved. Nevertheless I would expect at least the full 2D/3D case (as sketched above) to work.