`build_stat` ignores unknown axes but deletes previous definitions

**Describe the issue**
If a gftools build config.yml does not contain STAT information, `build_stat` is called, which ignores unknown axes (e.g. `ROND`), while deleting previous STAT entries. The table is then incomplete (and in my case, mangled, because suddenly wght style names are used as axis names, but that may be a different bug).
