[DF] Add support for 2-dim, multi-dimensional arrays

[martamaja10]

A recurring feature request. TTreeReader cannot read branches that contain multi-dimensional arrays (or at least not as such, in some cases it might be able to read a flattened version). This means that we cannot read certain branches from MiniAODs.

See also https://sft.its.cern.ch/jira/browse/ROOT-9509 .