Metadata coherence AccesMode

Please provide the following details if possible or relevant:

- [1.0.2] the version of Ace impacted by the issue (as returned by the `ace -v` command).
- [v8.7.0] the version of Node used to run Ace (as returned by the `node -v` command).
- [MacOS 10.3.6] your operating system and version.
- [] if the issue is a bug when running Ace: the error message as produced in the log file (you can enable debug logging with the `--verbose` option).
- [] if the issue is about the HTML report: the browser(s) and version(s) you used.
- [] if possible, let us know if you can share a sample input file to help us reproduce the issue.

Ace should verify coherence between accesMode and EPUB content.
If an EPUB has images, an accessMode visual must be present in the metadata.
`visual`