Update Doxygen documentatuion of romb() method.
Implement adaptive Simpson's rule integrator.

A new integrator has been implemented together with a unit test.
Update Doxygen documentation.
Implement Gauss-Kronrod quadrature in GIntegral class.
Set also relative error if information is available.
Romberg integrator now also sets relative and absolute error.
Add shell and disk model test data.
Implement and optimize stacked cube response computation for radial models.