[1.3] Good gui MetaShARK

This update comes with enhanced GUI. Main new features or key development steps are:

• the merge of the Custom Units step in the Attributes one
• the switch of .RData to .JSON format for save files (which makes the app >100x faster !)
• the addition of a progress bar in Fill EML.
• the possibility of writing EMLdown, which is a pre-rendered HTML format for a data paper
• many bug fixes

The app is still in development. However, the more tests are led, the more it will answer the users' needs. Do not hesitate to spread the word !

Upcoming development:

• Users session in server mode (ORCID-based authentication)
• Non-table data files support
• EMLdown export to PDF and commoner format
• Better app code integration for sustainability purposes.
• Ontologies and terminologic resources access