Reorder dimensions to start with space and time

[niksirbi]

Is your feature request related to a problem? Please describe.
In our current dataset structure the order of xarray dimensions is: time, individuals, keypoints, space.

This is not very intuitive, namely because time and space are the most essential dimensions which should always exist regardless of how motion was tracked. As we move toward supporting more types of data, and possibly making non-spacetime dimensions optional (e.g. create keypoints only if more than 1 is present, and ditto for individuals), and it makes more sense to put the "essential" dimensions at the beginning and the optional ones at the end.

This was brought up by @sfmig, and we both think this would make sense.

Describe the solution you'd like
Reorder dimensions in order of "essentiality": time, space, keypoints, individuals, views (if we adopt #196).
This shouldn't be too hard as we could simply add xarray's transpose() methods at the end of our loading functions to get the desired order. But we would have to adapt validators (related to #210), tests and docs.

Describe alternatives you've considered

• space, time, keypoints, individual, views
• Keep things as they are, don't fix it if it ain't broken?

Additional context

• Discussions with @vigji on essential vs optional dimensions.
• Support for multiple views issue and PR Add the option for multiple views loading #197
• On re-designing the validators: Need to re-design dataset validators? #210

[vigji]

I think this would be important yes! Even with the option of optional dimensions, keeping the essential first would be very relevant