CPO documentation

[mb706]

• work in progress -

[mb706]

Current status of the tutorial can be seen here.

I have now made the tutorial imho long enough to be informative and short enough to be interesting for someone learnign mlr in combination with CPO. I am not mentioning retrafo() and inverter() much, since attaching a CPO to a Learner does all the retrafo-related things internally, so I don't think it should be in an introductory tutorial.

I also mention the vignette which goes more in-depth.

When more things in mlr get replaced by mlrCPO equivalents the respective tutorial pages could also be updated.

[mb706]

(Travis build script currently pulls the paramSetSugar branch of ParamHelpers)

[larskotthoff]

So is this ready to be merged now?

[mb706]

I'm not sure if this should be merged before mlrCPO is on CRAN (otherwise I could adjust the introduction that mlrCPO should be installed from github for now; using the paramSetSugar branch of ParamHelpers in the travis script also a crutch that would need fixing first), but I think the document is otherwise "done". I welcome feedback on whether I should go more or less in detail at points.

[larskotthoff]

Ok, please ping when everything is ready!

[pat-s]

@mb706 Can you port this PR to the pkgdown files?