add partial count redux (adapted patch from Ricard Marxer)

doc/snippets/PartialRedux_count.cpp

@@ -0,0 +1,3 @@
+Matrix3d m = Matrix3d::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here is the count of elements larger or equal than 0.5 of each row:" << endl << (m.cwise() >= 0.5).rowwise().count() << endl;