Nicer `__str__`

[AdamHillier]

At the moment, if a component appears multiple times in the nested component hierarchy, it will get printed multiple times in __str__. It would be nice if it would only be printed once, ideally at the 'highest' level.

[AdamHillier]

It would also be good to get some nice colour coding which shows which parameters have been overriden through the CLI.

[jneeven]

As we discussed, it would also be nice to add a boolean flag to each Field that determines to what extent the Field is printed, e.g. expand_print=True. Then we can set it to False for things we don't want to have in the parameter overview, e.g. printing attributes of the type tfds.core.DatasetInfo is very verbose and also unnecessary; simply printing the name of the class would suffice.