Minor refactoring and cleanup

- Rename ContextIndex.keys to contexts
- Rename  TextIndexer.keys to contexts
- Export checkWith and extractTerms in Grammar again
- Comments
Add member function to ContextIndex
InsertContext throws an error if the context already exists
Add test for inserting an existent context
Remove old Server.Interpreter
Change context weight from Int to Float and add test
Fix potential bug: toDocAndWords requirement unchecked due to laziness
Remove IndexMetadata of ApiDocument which contained an Analyzer option

- Analyzer is set by the schema
- Individual transformations can be done by transforming the testdata
  or directly using the WordList (Map Word [Position])
Cleanup