Add :let and :unlet commands to create and remove local variables

notebooks/tests/let.ipynb

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

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

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

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