Standardize meta mandatory key

[AlexVCaron]

Bug category

• Critical (some functionalities is not working at all)
• Major (something is not working as expected)
• Minor (something but could be improved)
• Trivial (documentation needs correcting and other non-functional issues)

Describe the bug

While working on #175, I mixed optional and mandatory fields between input and output definitions in the meta.yml. The metadata was still valid (optional: true equals mandatory: false), but the conversion scripts don't support switching between the two conventions, so the information was missing from the API on the website.

Describe the solution

I chose mandatory as the standard key for both input and output, and dropped optional completely. Modules were already configured as such, but not Subworkflows. While I know optional makes more sense when talking of outputs, having to switch when writing the meta error prone (I think I make a great example of it 😅). BTW, I have no opinion on this really, so if optional is preferable, I'll be happy to change. I do think we need to keep only one though.

[arnaudbore]

GTG