Macro hyperopt::impl_multivariate

macro_rules! impl_multivariate {
    ($($index:tt $type_:ident,)+) => { ... };
}

Implement Density, Sample, and Kernel for a generic tuple of kernels.

Due to the macro syntax limitation, one also has to specify indices manually like so:

impl_multivariate!(0 K1, 1 K2, 2 K3,);