Problem recovering tree histogram when files have been split

[jgalan]

File splitting was introduced at #190 and #201.

I found out that when the files are split I am not able to use the following to recover the generated histogram

run0->GetAnalysisTree()->Draw("anaTreeVar");
TH1D *h = (TH1D *) run0->GetAnalysisTree()->GetHistogram();

When the output has been split, i.e. when we find .root, .root.1 ... .root.N the resulting pointer h will be a nullptr, but it will be valid if the file was not split, by just increasing fileSplitSize value (for example).

I wonder if this has an easy fix.