Dataset join

[mqzhang]

Add a join function for Dataset:

a = {'a' => [1,2].to_vector, 'b' => [2,3].to_vector}.to_dataset
b = {'c' => [1,2].to_vector, 'd' => [5,6].to_vector}.to_dataset
c = {'a' => [1,2].to_vector, 'b' => [2,3].to_vector, 'd' => [5,6].to_vector}.to_dataset
p c == a.join(b,['a'],['c']) # => true

[clbustos]

Thanks! If you can add the test, should be great.

[mqzhang]

@clbustos Thanks to accept it. Sorry I don't know how to add the test. I used an external script with a monkey patch to test the function.