📰 Custom Issue

The Connectivity class has many methods/properties that include src or tgt in their name - respectively used to refer to the left (face in face_node) and right (node in face_node) elements of a UGRID Connectivity. We chose src and tgt to take advantage of their already established use in regridding, but unfortunately we now have confusing cases where these Connectivity properties are used in regridding code (e.g.)! So we need to use new words that are also adequately descriptive and don't clash with other parts of the codebase.

• Agree on more appropriate replacements for src and tgt.
  My personal preference is lhs and rhs, since trying to describe the actual roles in short words is overly difficult.
• PR replacing all uses in Iris.
• Follow-up PR in iris-esmf-regrid, since this contains downstream references to methods/properties that will be changed.