Add helper functions to identify types of metrics / metric sets

[EmilHvitfeldt]

for use as is_class_metric() to determine if accuracy() is the right thing. We have an as_tibble() for metric_sets

yardstick/R/aaa-metrics.R

Line 317 in be744a3

as_tibble.metric_set <- function(x, ...) {

but it is not documented well enough