rootio/cmd/root-gen-datareader: handle branches differing by case
rootio/cmd/root-gen-datareader: handle branches differing by case

Branches of a ROOT TTree may be named like so:

  aBranch float64
  ABranch float64

For ROOT, they are different, valid branches.
This confused root-gen-datareader event struct generation code that
would generate:

  type Data struct {
      ABranch float64 `rootio:"aBranch"`
      ABranch float64 `rootio:"ABranch"`
  }

(to honour Go export rules and reflection rules)

So, instead, prepend "ROOT_" to each branch name and keep the branch
name intact:

  type Data struct {
      ROOT_aBranch float64 `rootio:"aBranch"`
      ROOT_ABranch float64 `rootio:"ABranch"`
  }

go-hep/hep,go-hep/hep