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.
Note on eras: I've added every decade since flight started for
consistency and completeness. For example, the Bell X-1 is from
the 1940s but is post-WW2.
- extract per-variant thumbnails from -set.xml files
- handle multiple primary aircraft in a directory (this will
need client-side changes too, eventually)
This restores the ability to use includes in -set.xml files visible
to the catalog code, and also exposes some problems / validation
issues in our -set.xml files. (Which can of course be fixed)
