Define run_pre() and run_post() for use in the repl

[aroberge]

Currently, the "pre" code is automatically executed when a world is loaded with the repl activated. For more flexibility, this should be changed and the user given the control to do run_pre() and run_post() when needed.

When this is done, "Done" should be also redefined so that it does run_post() automatically as well.

Finally, the "world examples" tutorial will need to be corrected as it describes the current default.

[aroberge]

The equivalent to run_post() is already include in done() when running from the repl. Since this is the case, there is little reason not to do run_pre() automatically when loading a world from the repl.