added two tests with optional
fixed strange behaviour in evalIsNullNotNull, when evaluating atoms of the form IS_NOT_NULL()
Merge branch 'feature/aggregates2' of github.com:ontop/ontop into feature/optional-union2
a tricky fix in pushTypes
Merge remote-tracking branch 'origin/feature/new-type-pushing-v2' into feature/optional-union2

Conflicts:
	reformulation-core/src/main/java/org/semanticweb/ontop/owlrefplatform/core/unfolding/DatalogUnfolder.java
Merge branch 'version2' into feature/optional-union2
Bad *IMDB*.odba filename
Merge branch 'feature/optional-union2' into version2