Remove warnings about rebinning

[eguiraud]

Currently ROOT rightly produces warnings when we call Rebin(2) on histograms with an odd number of bins (in fact all histograms have 25 bins).

In the reference implementation the bins before coordinate=110 are cut before the rebinning, so only 20 bins remain which can correctly be rebinned 2 by 2.

We should do the same in plotting.py but I'm not sure what's the best way to slice a histogram axis (in UHI notation it would be h[loc(110)::rebin(2)]).