Fixes for demos - all collected in rosworld21 branch for now:
- Allow metadata to be serialized into a string, for use by storage
- emersonknapp/string-metadata (metadataio)
- Don't preprocess bag files twice - fix crash on file compression
- emersonknapp/no-preprocess-twice
- Present converter plugin choices correctly
- emersonknapp/converter-plugin-choices
- Give introspection TS to the converter
- emersonknapp/converter-needs-introspection-ts