Make Command.inspect a required method

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

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

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

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

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

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

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

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

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

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

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