Expression attributes (variables) are now shared among expressions

src/main/java/avatartranslator/AvatarActionAssignment.java

src/main/java/avatartranslator/AvatarActionOnSignal.java

...main/java/avatartranslator/AvatarExpressionAttribute.java

src/main/java/avatartranslator/AvatarExpressionSolver.java

src/main/java/avatartranslator/AvatarSpecification.java