Merge implementations of :constants, :init and :exec

notebooks/tests/animate.ipynb

src/main/java/de/prob2/jupyter/ProBKernel.java

...main/java/de/prob2/jupyter/commands/ConstantsCommand.java

src/main/java/de/prob2/jupyter/commands/ExecCommand.java

...ain/java/de/prob2/jupyter/commands/InitialiseCommand.java