linter: warn if expect_num_outputs is missing

[bernt-matthias]

if outputs have filters or are discovered

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[bernt-matthias]

Can you push this into a profile version ? I don't think this should be a warning for old tools.

Hrmm. This change should not affect the functionality of tools, but just the linter. Which is as far as I see only called from planemo... imho the linter should always output according to the latest standards which allows tools to slowly catch up (always if they are touched).

[nsoranzo]

@bernt-matthias If ready for review, please remove the status/WIP label.