Add placeholders for missing methods in (Let|Unlet)Command

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

@@ -49,6 +49,12 @@ public final class LetCommand implements Command {
 		return null;
 	}
 	
+	@Override
+	public @Nullable DisplayData inspect(final @NotNull String argString, final int at) {
+		// TODO
+		return null;
+	}
+	
 	@Override
 	public @Nullable ReplacementOptions complete(final @NotNull String argString, final int at) {
 		// TODO

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

@@ -49,6 +49,12 @@ public final class UnletCommand implements Command {
 		return null;
 	}
 	
+	@Override
+	public @Nullable DisplayData inspect(final @NotNull String argString, final int at) {
+		// TODO
+		return null;
+	}
+	
 	@Override
 	public @Nullable ReplacementOptions complete(final @NotNull String argString, final int at) {
 		// TODO