Variable resolution fieldsfiles

This PR is a result of my review of #619. It squashes those commits into one, and then adds my own commit with the review actions on top.

The purpose of the PR is identical to that of #619, except it removes the need for real test data, and removes the warning that the cross reference module is not editable (that is for another discussion in iris-code-generators - and once we have an automatic way of producing the file, I'm :+1: for adding the "don't edit" message back).