fix to avoid slow repr generation during interpolation

User testing has identified an interpolation performance issue that has been introduced since 1.6. One of the reasons for this is the use of `list.index()` on a list of coords. It turns out that if the coord is not in the list Python raises a ValueError (as expected and handled by the Iris code), but this in turn uses `coord.repr()` which is relatively a slow operation due to the string operations. This PR fixes the issue.