Make agtboost model object entirely numerical

[Blunde1]

The agtboost model class contains the type of loss-function as an std::string type attribute. This is problematic (for memory and garbage collection reasons) when working with a model object from the R side. See e.g. the following comment https://github.com/Blunde1/agtboost/issues/55#issuecomment-1192825407.

The model could and should consist of entirely numerical attributes.