get_best_mapping: get_best_mapping
In cvraut/viLDA: variational inference Latent Dirichlet Allocation

returns the best mapping through complete search

1	get_best_mapping(predicted,true)

`predicted`	n length vector of predicted groups
`true`	n length vector of true groups

list("prop_correct","mapped_pred","true_val","mapping")

$prop_correct: float 0-1 proportion of the true value, that best prediction map correctly gets
$mapped_pred: n length vector, mapped predictions of best mapping
$true_val: n length vector of the true value
$mapping: k length vector (k is the number of groups) of the best group mappings