Make clone_tree() return the tree rather than write it

[masonproffitt]

No description provided.

[masonproffitt]

The original idea behind this was to attempt to recreate the return value of TTree::CloneTree(). Considering how uproot deals with writing files, I don't think it actually makes sense to retain this. The tree object used for writing is completely different from the object of a tree that has been read from a file. In order to preserve the symmetry of the interface of the new tree, you'd have to close the new file and then read it in. I think that's too much extra work to stuff into clone_tree() since the return value will likely often be ignored.

[masonproffitt]

New strategy: dump all the responsibility of writing tree-like data into write_tree(). This frees clone_tree() to just return some representation of the tree that doesn't have to be stored in a file (yet). This accomplishes the original goal of trying to more closely align with the behavior of ROOT's TTree::CloneTree(). The most logical return value types are dict or Table (in awkward0, otherwise RecordArray in awkward1).