has_class: Returns TRUE if data.frame columns have a specified class
Description
This is meant to be used with `assertr`'s `verify` function to check
for the existence of a specific column class in a `data.frame` that is
piped to `verify`.
Usage
has_class(..., class)
Value
TRUE if all classes are correct, FALSE if not
Arguments
...
An arbitrary amount of quoted column names to check for