Update of documentation for rose-stem

This adds a proper user guide and advanced tutorial for rose-stem. The user guide should enable users to run rose-stem, and to have a vague idea of how it works.

The advanced tutorial guides the developer through creating a basic rose-stem suite (nothing fancy), but it should provide a good jumping off point for people developing their own suites.

This change also adds the --group option as a synonym for --task, which I think is less confusing (--task does not specify a cylc task!); for the moment both will work until we get agreement on the change.

@benfitzpatrick, when you get a spare hour or so would you mind reviewing? I've gone through your feedback on the draft and implemented that, and Paul C has looked through it too, so hopefully it won't be too bad. It's not urgent that it gets in the next Rose release, although it'd be nice to have it at least committed before the next training course so I can talk about the changes! Thanks!