Add custom histogram binning support

[AxelPuntke]

This update allows to use a non-uniform binning for the histograms, e.g.

task->AddH1({"y", Variable::FromString("Candidates.rapidity"), {6, (Double_t[]){0, 0.6, 1.0, 1.5, 2.0, 2.5, 4.0}}});
task->AddH2({"y", Variable::FromString("Candidates.rapidity"), {6, (Double_t[]){0, 0.6, 1.0, 1.5, 2.0, 2.5, 4.0}}}, {"p_{T} [GeV/c]", Variable::FromString("Candidates.pT"), {8, (Double_t[]){0, 0.2, 0.4, 0.55, 0.85, 1.2, 1.5, 1.9, 2.5}}});

It would be nice to have it included into the next version.
In the current implementation the behavior is that the range of the custom binning is used, but all bins have uniform width (range/number_of_bins), which took me some time to realize.