Fix reading OPL files when filename attribute is not present, add corresponding unit test, update corresponding schema

Pierre Dell'Ovarequested to merge
20-error-reading-opl-files-which-does-not-contain-filename-attribute into main

Fix reading OPL files when filename attribute is not present, add corresponding unit test, update corresponding schema

Please see my comment on the related issue

Closes #20 (closed)

Merge request reports