Component __repr__ and __str__

[wpbonelli]

There are a few issues with these right now

• Hide the datatree. Currently included, it's often just a big wall of text and should be omitted. Plus the user can always print the datatree if they want it. Make data tree an attrs attribute wpbonelli/xattree#18 should fix this if we use repr=False for the hidden datatree field.

• The default attrs repr isn't bad but we might want a more condensed representation?

[wpbonelli]

Just defer to attrs and xarray independently on this? Closing until there is a demonstrated need for custom reprs