-add set...() methods and their documentation; remove Generic

-use row-major traversal when the number of columns is fixed
 and the number of rows is dynamic
-other minor changes

doc/snippets/MatrixBase_setOnes.cpp

@@ -0,0 +1,3 @@
+Matrix4i m = Matrix4i::random();
+m.row(1).setOnes();
+cout << m << endl;