For GP, the default value is: ``model::GP<Params, kf_t, mean_t, opt_t>>``,
- with ``kf_t = kernel::SquaredExpARD<Params>``
- with ``mean_t = mean::Data<Params>``
- with ``opt_t = model::gp::KernelLFOpt<Params>``
(meaning: kernel with automatic relevance determination and mean equals to the mean of the input data, that is, center the data automatically)
For Statistics, the default value is: ``boost::fusion::vector<stat::Samples<Params>, stat::AggregatedObservations<Params>, stat::ConsoleSummary<Params>>``