Full package docs
Fix missing S4 class error for WKNN docs

* adding -class to the name seems to fix this.
fix non-ascii char in package docs
better style for makevars (+=_)
Don't @export c functions directly

* Use custom R wrapper based on original RcppExports.R
* will actually ignore the R wrapper functions
Just use a single knn_generic C++ func
refactor knn(_brute) R funcs to use knn_generic
minor fixes knn docs
retire knn_brute function
fix: buildtree honours requested treetype
Handle choice of searchtype in C++

* this saves about 4us on my machine!
  (when 300 x 300 3D points, k=1 takes about 180us)