Vtreat

* Control imputation on design_missingness_treatment.
* Share cross-splits in multinomial mode to minimize data leakage.
* Earlier argument checking.
* Default classification target to TRUE.
* Some documentation improvements.
* Move wrapr to Depends.

* Check for same-frame on prepare paths.
* Pipe into ft.
* Rename multinomial arguments to match other methods.
* Move parallel to a suggestion and configure a global option to shut it down ( vtreat.allow_parallel ).

* Fix multinomial variable restriction.
* Fix FT multinomial columns copy.
* Work on ft serialization environments.
* More tests.
* Improve custom coder flexibility.

* Center/scale improvement from nfultz.
* Documentation updates.
* fit_tranform interface.

* fix wrong y-name in multinomial cross frame.

* Some doc updates.