Offer through API calculations using groupby. (Implement groupby in API)

Could be a good idea implement a grupby inside of API module, this allow us to use methods on each group. 

The main idea is can do:

```pyhton

import ineqpy as inq
data = inq.api.Survey(data, weights='w', group='county')
data.groupby(data.county).gini('income')
data.groupby(data.county).atkinson('income')
```

and then obtain the gini or atkinson index by county.
