Support various methods for Group objects

[miladh]

Properties in h5py that we could support:

• [x] atters
• [ ] clear
• [x] close
• [ ] copy
• [ ] driver
• [ ] fid
• [ ] file
• [x] filename (directory)
• [ ] flush
• [x] get
• [ ] id
• [x] items
• [x] keys
• [x] mode (io_mode)
• [ ] move
• [x] name
• [x] parent
• [ ] pop
• [ ] popitem
• [ ] ref
• [ ] setdefault
• [ ] update
• [x] values
• [ ] visit()
• [ ] visititems()
• [x] iter()
• [ ] Intermediate groups can be created automatically "grp = f.create_group('foo/bar/baz')"
• [ ] __str__()
• [ ] __repr__()
• [x] __len__()