Enhance AST with Feature Mappings

[tcerny]

Description

Every AST should contain a variable which contains a logical statement for representing its feature mapping. This requires to use the prop4j library which can be found here: https://github.com/FeatureIDE/FeatureIDE/releases/download/v3.7.0/de.ovgu.featureide.lib.fm-v3.7.0.jar
Example usage:

   Node myFormula;
   Literal a = new Literal("A");
   Literal b = new Literal("B", false);
   myFormula = new And(a, b);

Definition of Done

• AST class contains the Node of the prop4j lib as class variable
• The usage of this variable is tested properly
• CI does not fail

[wurstbroteater]

TODO Links this to Pull request which will later adress (among others) issue #54