HTML repr : Increase width + fold all by default?

[jbusecke]

I think the HTML repr could be made even more useful by these changes:

1. Increase the width. I often encounter situations like this

where the Dataset repr is very compressed, but there is still a lot of width space in the cell. Would it be possible to use the full width for the repr?

2. I believe datatree adopts the xarray default for folding (e.g. attrs are folded, coordinates are not, data variables depend on the number of variables?). I believe that in the tree case it would be easier to navigate medium to large trees with everything folded by default. I am not sure if that can easily modified but I believe it would help with the UX a lot.

[TomNicholas]

Thanks for raising these Julius. However I think we already have issues tracking these improvements (1. in #91 and 2. in #145), so I'm going to close this in favour of those.