Add summary of ugrid to cube print out

[lbdreyer]

DO NOT MERGE: Blocked by #3684

NOW READY FOR REVIEW

This extends the cube repr to include ugrid information.

• In the cube header, the dimension that is mapped by the unstructured mesh is identified with a preceding *.
• A new section (ugrid information) is added.

The repr looks like:

air_potential_temperature / (K)     (-- : 1; full_levels: 39; *-- : 864)
     Dimension coordinates:
          full_levels                   -               x         -
     Auxiliary coordinates:
          time                          x               -         -
     ugrid information:
          Mesh2d_full_levels.face       -               -         x
          topology_dimension: 2
          node_coordinates: longitude latitude

And the repr htm looks like

Important things to note:

• Previously we had discussed including a vertices per face: min 4 max 4. I decided against this as to compute it, we need to load the data from the face_node_connectivity variable (if it exists) and considering this may be big I thought it wasn't worth the performance cost
• I haven't added tests (other than extending a test added in Ugrid cf load #3684). Our testing of cube print summaries has always been pretty limited and I thought that since we are working with PoC code it may not be necessary?? But happy to add some test if you think that would be best

[pp-mo]

Hi @lbdreyer this should benefit from rebase, since #3685

[pp-mo]

I can't help feeling we should be applying the same here as in Iris --> standard_name or long_name or var_name.

[lbdreyer]

Yeah, I was a bit unsure about this bit. I'll do the different naming as you suggest.

[trexfeathers]

Looks OK, works fine on a number of files I tried, thanks @lbdreyer! Please could you address / push back on a few code layout things I've highlighted?

[pp-mo]

Just taking a quick look.
If you can manage it, I do think a simple testcase example would be useful -- probably a quick integration test with the existing file.
How about : lbdreyer#8
?

[pp-mo]

Just taking a quick look.

Otherwise, only skimmed but LGTM : the additions to ugrid_cf_reader seem fine.

[lbdreyer]

Thanks @trexfeathers and @pp-mo for the review! I have dealt with your comments in the most recent commit.
Please re-review

[trexfeathers]

Thanks for addressing @lbdreyer, love the improvement to vector_summary 💖.
Won't merge this myself since it's probably best that @pp-mo checks the names change, given he originally caught that and I didn't.

[pp-mo]

TBH this code area is a bit nasty, and I really don't envy you doing this, so 🥇 for taking it on @lbdreyer !!
As I've no special understanding here, I feel I could easily still be missing something, but it clearly functions, so I think we should bank it + deal with further problems if+when they arise.