temporary solution class constraint
Merge remote-tracking branch 'origin/dev' into erik

Conflicts:
	gs-kernel/src/main/java/org/genericsystem/kernel/AbstractVertex.java
	gs-kernel/src/main/java/org/genericsystem/kernel/systemproperty/constraints/InstanceValueClassConstraint.java
Instance Value class constraint ok
imporve Singular constraint for all positions
Merge remote-tracking branch 'origin/dev' into erik
check constraint on holders
change a visibility
Merge remote-tracking branch 'origin/erik' into dev