Wildcard support for trees

[alexander-held]

In a typical ATLAS workflow, files may contain many trees that are built with varying detector configurations (and then used for systematic uncertainties):

file.root
  - nominal
  - variation_1 
  - variation_2

It would be convenient to support wildcards for tree names to avoid having to process each tree specifically.

I am not sure how to best organize multiple trees attached to a single dataset, since they contain different information. One possibility might be to build the dataset name from the -d option plus the tree name. All datasets belonging together could be identified with additional metadata. Another option might be adding another layer, where the dataset can contain multiple "sub-datasets", which then each correspond to a specific tree.