Store results of evaluating validating expressions
A object of class validation stores a set of results generated by
evaluating an validator in the context of data along with some
metadata.
Other validation-methods: aggregate,validation-method,
all,validation-method,
any,validation-method,
barplot,validation-method,
check_that, compare,
confront,
plot,validation-method,
sort,validation-method,
summary, values