Implement "assert_rows"

[tonyfischetti]

assert_rows :: data.frame df => df -> (df -> [a]) -> (a -> Bool) -> df

assert_rows(a, b, c, ...)
a = data frame
b = function that takes data frame and returns nrow vector
c = predicate
... optional "select" rows

map the predicate c over return value from b(a)
[map(c), b(a))]

example:

iris %>%
   assert_rows(cosine_dist, within_bounds(0,.5), -Species) %>%
    ...

the error message will read like....
Error: Assertion 'within_bounds' violated at row 4 of iris (value: 32.4)