Replace Apache Commons Lang with standard library solutions

README.md

build.gradle

de.prob.core/.classpath

de.prob.core/META-INF/MANIFEST.MF

de.prob.core/src/de/prob/core/ProblemHandler.java

de.prob.core/src/de/prob/core/command/ComposedCommand.java

de.prob.core/src/de/prob/core/domainobjects/Operation.java

de.prob.core/src/de/prob/core/domainobjects/Variable.java

...rob.core/src/de/prob/core/langdep/EventBAnimatorPart.java

...core/src/de/prob/eventb/translator/ContextTranslator.java

...c/de/prob/eventb/translator/internal/ModelTranslator.java

de.prob.ui/src/de/prob/ui/historyview/HistoryView.java

...c/de/prob/ui/stateview/statetree/StateTreeExpression.java

...src/de/prob/ui/stateview/statetree/StateTreeVariable.java