Dev/anyof etc

[magdanowak]

Some details:

• as originally discussed, we don't yet deal with schema references, so NadiaCombinedSchema doesn't recognize and follow referenced schemas,
• which is why the schemas are called schema_i in case of validation failure - not the most elegant solution, but maybe some other ideas during review?
• the code doesn't actually work correctly because of conflicts with additionalProperties implementation (mentioned by @magdanowak in #5), so merging should probably wait until these are resolved
• this update touches parts of codebase without unittests, so that's something for the future

[codecov-io]

Codecov Report

Merging #9 into develop will decrease coverage by 4.6%. The diff coverage is 20.68%.

@@             Coverage Diff             @@
##           develop       #9      +/-   ##
===========================================
- Coverage    95.58%   90.97%   -4.61%     
===========================================
  Files           18       18              
  Lines          385      410      +25     
===========================================
+ Hits           368      373       +5     
- Misses          17       37      +20

Impacted Files	Coverage Δ
nadia/schema.py	27.27% <20%> (-15.59%)	:arrow_down:
nadia/api.py	35.29% <21.42%> (-21.85%)	:arrow_down:

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[dexter2206]

Great job, I agree we should probably wait until additionalProperties issue is fixed (this, in turn, should wait until possible refactor following from #6).

As for the references, they should be automatically handled by ymlref. However, there is no way of extracting schema names using ymlref for the time being. I'll see if it can be somehow easily implemented.