Minor fixes for allOf and other improvements.

[altro3]

• Changed context.fail to context.warn when path variable is not found (!critical because now if pathVariable is not found the build will fail)
• Fix duplicates references in allOf block
• Sorting schemas in allOf block: schemas with ref must be first

[altro3]

@graemerocher Hi! Could you merge this PR and #982 and finalize release?

Also, please, close next tickets:

Fixed: #977, #104

Won't do: #944, #904

Not a bug: #974

[graemerocher]

@altro3 as a contributor you should be able to close those tickets

[altro3]

@graemerocher Hi! Thank you for your trust! Yes, now I can do it myself :-). Please don't create a new release just yet. I found a few more small inaccuracies, I would like to fix them before the release is finalized