Phase out slice for dense matrices (#2259)

* better documentation, test against eigen slicing

* remove a bunch of slices

* clean up templates; static asserts on vector types

* final purges of slice

* fix templates in debug and ears test

* fix slice mask bug

* fix slice bug

* boo. try to get around windows poor template deduction

* annoying left over bad windows ❄️ template

tutorial/406_FastAutomaticSkinningTransformations/main.cpp

@@ -1,6 +1,5 @@
 #include "precomputation.h"
 
-#include <igl/slice.h>
 #include <igl/readDMAT.h>
 #include <igl/readOBJ.h>
 #include <igl/arap.h>
@@ -147,7 +146,7 @@ int main(int argc, char *argv[])
   // Plot the mesh with pseudocolors
   igl::opengl::glfw::Viewer viewer;
   viewer.data().set_mesh(U, F);
-  viewer.data().add_points(igl::slice(V,b,1),sea_green);
+  viewer.data().add_points(V(b,Eigen::all),sea_green);
   viewer.data().show_lines = false;
   viewer.callback_pre_draw = &pre_draw;
   viewer.callback_key_down = &key_down;