3.0 map reduce

This adds documentation for the `mapReduce` method. I decided to only show examples of changing fundamentally the result set and not just a "mapper". After writing this, I realized that it is important to write a `Query::format()` function or similar, something simpler than MapReduce but that does not force you to execute the query like we have now with the collection methods.

TODO:

* [ ] Explain that map reduce does not execute the query yet
* [ ] Show examples with chaining more query methods after map reduce
* [ ] Explain that it is possible to chain more than one mapReduce routine
* [ ] Add warning on changing the selected columns in stacked finders. Teach how to erase all mapReduces operations.