rootio: add support for TChain

it should be possible to combine multiple rootio.Trees (with the same layout) from multiple ROOT files into one rootio.Tree.

right, now, to get at a rootio.Tree, one does:

func main() {
	f, err := rootio.Open("testdata/chain.1.root")
	if err != nil {
		log.Fatal(err)
	}
	defer f.Close()

	obj, err := f.Get("tree")
	if err != nil {
		log.Fatal(err)
	}
	tree := obj.(rootio.Tree)
	fmt.Printf("entries= %v\n", tree.Entries())
}

the rootio.Tree interface is:

// Tree is a collection of branches of data.
type Tree interface {
	Named
	Entries() int64
	TotBytes() int64
	ZipBytes() int64
	Branch(name string) Branch
	Branches() []Branch
	Leaves() []Leaf

	getFile() *File
	loadEntry(i int64) error
}

it should be possible to have a type that implements the rootio.Tree interface but logically concatenates multiple rootio.Trees together. Something like the io.MultiReader function: https://godoc.org/io#MultiReader

Initially, one could just require the user to pass already created/retrieved rootio.Tree to a, e.g., Chain function:

// Chain returns a new Tree that is the logical concatenation of all the input Trees.
func Chain(trees ...Tree) Tree { ... }

one should think about resource ownership:

• who should close the ROOT file containing a Tree when that Tree has been completely read ?
• how to integrate the chain type with the Scanners ?
• is it better to have a Chain function that takes a tree name and a list of file names ? (so resource ownership is clearer). it's less composable...
• perhaps introduce a ChainScanner that opens and closes files as needed when each Tree has been consumed ?