Add support :let variables to more commands

notebooks/tests/animate.ipynb

notebooks/tests/assert.ipynb

notebooks/tests/dot.ipynb

notebooks/tests/let.ipynb

notebooks/tests/solve.ipynb

notebooks/tests/table.ipynb

notebooks/tests/type.ipynb

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

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

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

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

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

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

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

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

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

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

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

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

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