Determine if an object is a valid tree prior
See also
tree priors can be created by create_tree_prior
Examples
check_empty_beautier_folder()
is_tree_prior(create_bd_tree_prior())
#> [1] TRUE
is_tree_prior(create_yule_tree_prior())
#> [1] TRUE
!is_tree_prior("nonsense")
#> [1] TRUE
check_empty_beautier_folder()