Format repr_html for better readability

[ESadek-MO]

See https://github.com/SciTools/iris/pull/6476.

One of the things noticed when @stephenworsley was working on this is that the html can be slightly hard to read, and in some places the table cells just cut off the text, rather than fitting to size.

I think this could be a rather straightforward change, by adding an auto width formatter, and aligning all text to centre (which better maintains cell boundaries and therefore readability), but my experience is limited, so I might well be proven wrong here.

[trexfeathers]

but my experience is limited

You probably have more HTML under your belt than most of us!