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

add placeholders for some 'special topic' pages.

doc/I10_Assertions.dox

@@ -0,0 +1,11 @@
+namespace Eigen {
+
+/** \page TopicAssertions Assertions
+
+
+TODO: write this dox page!
+
+Is linked from the tutorial on matrix arithmetic.
+
+*/
+}