- move: DerivedTraits becomes MatrixBase::Traits

- the static constants are private again in the Derived classes
- more documentation and code snippets
- new isDiagonal() method

doc/snippets/MatrixBase_isIdentity.cpp

@@ -0,0 +1,5 @@
+Matrix3d m = Matrix3d::identity();
+m(0,2) = 1e-4;
+cout << "Here's the matrix m:" << endl << m << endl;
+cout << "m.isIdentity() returns: " << m.isIdentity() << endl;
+cout << "m.isIdentity(1e-3) returns: " << m.isIdentity(1e-3) << endl;