prior_mixture creates a mixture of prior distributions.
This is a more generic version of the prior_spike_and_slab function.
a list of prior distributions to be mixed.
a logical vector indicating which of the prior distributions
should be considered as a null distribution. Defaults to rep(FALSE, length(prior_list)).
a character vector indicating which of the prior distributions
belong to the same mixture component (this is an alternative specification to the is_null argument).
Defaults to NULL (i.e., is_null is used.