refactor: IWYU applied to the source files

`alternative_namespaces` examples left out as I consider them a subject for removal in the nearest future.

example/box_example.cpp

@@ -1,14 +1,40 @@
-#include <units/isq/si/constants.h>
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#include <units/format.h>
+#include <units/generic/dimensionless.h>
+#include <units/isq/si/amount_of_substance.h>
 #include <units/isq/si/area.h>
-#include <units/isq/si/speed.h>
+#include <units/isq/si/constants.h>
 #include <units/isq/si/density.h>
 #include <units/isq/si/force.h>
+#include <units/isq/si/length.h>
 #include <units/isq/si/mass.h>
+#include <units/isq/si/speed.h> // IWYU pragma: keep
 #include <units/isq/si/time.h>
 #include <units/isq/si/volume.h>
-#include <units/format.h>
 #include <cassert>
 #include <iostream>
+#include <utility>
 
 namespace {