Coords to print shorter, and avoid realising

[pp-mo]

Printing large coords has been a practical problem for some time : Although numpy can print arrays in a truncated form, the results for large coords are pretty unreadable (especially those with bounds)
-- and it always realises data.

This is really about Aux-coords, as only they have the potential to be multi-dimensional, and lazy.
Which, practically, means that only they will typically be "really large".
Nevertheless, it may be better to modify DimCoords also, for consistency.

Obviously, this is likely to be especially nasty for unstructured data.

We want to tackle this now, because it affects the printout of meshes -- which is currently pretty unreadable
However, it does mean "breaking change" -- if you care about the text output from printing coords.

STOP PRESS:
now also intending to produce a 'single-line format', inspired by the Cube.repr format.
This can help a lot with making Mesh printout more sensible.

[pp-mo]

Note: I think that this risks losing the 'special' support for date formatting in printing coord outputs, which would be a practical problem.
For this purpose, perhaps an abbreviated form can be used to retain that, and/or depend on size.

[pp-mo]

In case anyone doesn't get a ping from the Discussion, I just raised there an awkward question of column formatting ...
#4501 (comment)

Right now, having a discussion, a PR and an Issue all live on this one topic is looking positively awkard !