Improved use of UGRID terminology

[trexfeathers]

🚀 Pull Request

Description

Closes #4492. Altered variable names throughout Iris' UGRID support to make two changes:

• Use both element and location for more precision.
  • element = node/edge/face/volume
  • location = which element is the subject of this Cube/MeshCoord/Connectivity?
  • Enables the second change below
• Reworked the src/tgt terminologies in Connectivity to reduce name clashes and confusion.
  • src_location -> location_element
  • tgt_location -> connected_element
  • src_dim -> location_axis
  • tgt_dim -> connected_axis
    (previously discussed removal but found it was offering convenience in several places)

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Consult Iris pull request check list

[stephenworsley]

I think we're getting there, though I think there is still a bit of confusion in the documentation over location_element the type (which is what is stored in the attribute), 'location element' used to denote an individual element and a confusing use of location element to refer to the list of connected elements associated with that individual location element.

[pp-mo]

Some slightly random thoughts.

Raises some general points, which aren't necessarily best addressed here + now:

• not sure about our use of the word 'topology'
• still uncomfortable about the Mesh API + wish we weren't releasing this -- but it is experimental

[trexfeathers]

After discussion with @stephenworsley have proposed b7ec240 . The property is now named location.

• Consistent with the terminology in MeshCoords and Cubes.
• Differentiates between:
  • location - the string naming the element type e.g. face.
  • location element - an instance of the location e.g. a single face
• Allows documentation to be phrased like:
  
  • Avoids misleadingly referring to the attribute as an instance of the location when it's actually just the type of the location.
  • Still provides a link if the reader needs to learn more.
  • You can actually substitute the value of location into the sentence. So "... each face element ..." in the example above.

[stephenworsley]

Looks good, just a few minor comments about the documentation.

[stephenworsley]

Just one more tiny thing and I think we're good.

[stephenworsley]

Looks good.

[pp-mo]

Couple of minor readability points noted.
Otherwise reads really well now, I think.

[stephenworsley]

The new fixes look good, I'm happy to merge this now.