Dump output branch list with -O

Adds a command line arg -O to write a file (output-branches-[output-file].txt) containing the list of branch strings given to the TTree writer. Those are of the form "Container.branch -> tree_branch", with no modifications, and in the order that they were generated by minituple_config.py.

Needed now for manual validation. Could be formatted in a more friendly way but this is left for future work, as is adding a reference branch list and a check that the tests reproduce this (just for a near-maximal config).

The dump is enabled as a common option in easyjet-test.