Generate a Dict
Returns a generator spec producing a Dict with one generated value
per column. Each column’s generator draws one value;
na_prob controls the probability that any given column
value is NA.
columns (Dict): { name :: String :
gen_spec :: Dict } A Dict mapping column names to generator
specs.
na_prob (Float): = 0.1 Probability
of a column value being NA.
A generator spec.
prop_gen_dict([x: prop_gen_int_range(0, 100),
name: prop_gen_string_from("abc", 1, 5)])