Support various methods for Group objects

[miladh]

Properties in h5py that we could support:

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