adjust the API of KL methods for model selection

make the API work for `GradientModelSelection`
@karlnapf 
please merge this.
I try to set 
```
 SG_ADD(&m_lambda, "lambda",
"Dual parameter for variational s2",
MS_NOT_AVAILABLE, GRADIENT_AVAILABLE);
```
But it fails.

I guess that the reason is
```C++
double nlopt_function(unsigned n, const double* x, double* grad, void* func_data)
{
...
CMap<TParameter*, SGVector<float64_t> >* gradient=gradient_result->get_gradient();
	CMap<TParameter*, CSGObject*>* gradient_dictionary=
		gradient_result->get_paramter_dictionary();
...
}
```

which means all parameters in `m_gradient_parameters` will be tuned when `GradientModelSelection` is called.
However, variational parameters cannot be directly tuned via `GradientModelSelection`.
That is why I remove them from `m_gradient_parameters`