some simplification and reorganization for incremental stages
Merge branch 'master' into feature/stages

Conflicts:
	elk-reasoner/src/main/java/org/semanticweb/elk/reasoner/stages/IncrementalClassTaxonomyComputationStage.java
control incremental mode using setAllowIncrementalMode method of the
reasoner
+ access methods for TaxonomyState