…sure total ordering.

- The Operation classes now have a getChildren()
  method that returns non-owning pointers to all of their children

- Each Operation holds a vector of strings to store warnings that are emitted during the result computation. Those can be recursively retrieved
  using the collectWarnings() function that internally uses the getChildren()
  function mentioned above

- Out-of-vocab entries in value clauses previously triggered exceptions.
- Now rows that contain unknown words are ignored and trigger a warning in the result json.
- This is also tested in the end-to-end test

…abulary, because everything else leads to incorrect behavior.

The code is still somewhat ugly, but already much less uglier.

The external vocabulary was so far created using the ICU collation (or a wrong version of it bc.
of the "Externalization Prefix". This should be now fixed, but we still should refactor the Vocabulary to use proper strict and static typin

This now only fixes the unicode stuff.

…lVocabularyUnicodeFix

# Conflicts:
#	e2e/scientists_queries.yaml
#	src/index/Index.cpp
#	src/index/StringSortComparator.h