This will enable richer author meta-data in aircraft, and hence
nicer GUI presentation of author information. Client-side support
still to be added before this can be used
Support multiple primary aircraft in a package, which has several
aircraft developers have requested. This adjusts the handling of
the variant-of and primary-set tags.
Additionally, this adds partial unit-test coverage for -set.xml
scanning and catalog XML generation.
Run ./test_catalog to run unit-tests of the catalog generation.