Various documentation improvements, in particualr in Cholesky and Geometry module.

Added doxygen groups for Matrix typedefs and the Geometry module

doc/snippets/AngleAxis_mimic_euler.cpp

@@ -1,4 +1,4 @@
 Matrix3f m = AngleAxisf(0.25*M_PI, Vector3f::UnitX())
            * AngleAxisf(0.5*M_PI,  Vector3f::UnitY())
            * AngleAxisf(0.33*M_PI, Vector3f::UnitZ());
-cout << m << endl;
+cout << m << endl << "is unitary: " << m.isUnitary() << endl;