Adding __repr__ methods to Grid, Node and Finder

[ClementJ18]

I've always found the ability to just print an object very handy for debugging so I've taken the liberty of adding it to some of the key objects of the library.

[brean]

I am not so sure about this. datamodels __repr__ makes more sense for the Node class, although it might be a bit verbose.

[ClementJ18]

That's a good point. I missed the part that is is a dataclass on the master branch. On the pipy version it's just an object. I'll remove the __repr__ for Node, it doesn't have that many attributes so just using the built-in __repr__ is fine in my opinion.