Tree interface class (and various constructors)

[tomeichlersmith]

Have a Tree for holding all the Branches. Various constructors for the different access patterns.

• hdtree::Tree::load(filepath,treepath)
• hdtree::Tree::save(filepath,treepath)
• hdtree::Tree::transform(f,t,f,t)
• hdtree::Tree::inplace(filepath,treepath)

Add some syntax sugar for accessing/setting branch variables. e.g. tree["name"] = value sets the branch named "name" during that loop

Maybe some kind of tree.for_each so the tree.load and tree.save calls are done properly.