Better understand the output code of `TTree::SetBranchAddress()`

In !86 (merged), we have implemented Darwin::Tools::Flow to handle the input and output trees. We should try to better understand the output code of TTree::SetBranchAddress() in Darwin::Tools::Flow::GetBranchReadOnly(). The test on the existing branch might be redundant with the kMissingBranch return code. We should also better understand why kNoCheck is sometimes returned (although the rest of the code seems to run fine).