big improvements to tutorial, especially page 2 (matrix arithmetic).

add placeholders for some 'special topic' pages.

doc/I07_TopicScalarTypes.dox

@@ -0,0 +1,12 @@
+namespace Eigen {
+
+/** \page TopicScalarTypes Scalar types
+
+
+TODO: write this dox page!
+
+Is linked from the tutorial on the Matrix class.
+
+*/
+
+}