fixed related courses bug

* where there were two subject areas, related courses were wrong
Merge branch 'bugs/related-course-fix' of https://github.com/unikent/programmes-plant into develop
add a more complete test of merging related courses
add test for load_external_data
enhance test
Merge pull request #551 from unikent/refactor/add-to-api-tests

Enhance API testing to test related courses