McCabe Complexity

[This issue](https://github.com/openspending/openspending/issues/692) on another project puts it better than I can:

> McCabe complexity is a number that represents the number of different way one can traverse through a method. It is recommended that this measure never exceeds 10. Running a complexity checker on openspending results in 10 methods with too high complexity (current version). These should all be fixed to keep the code clean and maintainable.
>
> To see the errors I would recommend installing flake8 (can run McCabe's complexity checker and more), navigating to the root of the repo and running:

```
flake8 --max-complexity=10 --select=C distarray
```

the output is:

```
distarray/externals/six.py:559:1: C901 'print_' is too complex (22)
distarray/local/localarray.py:1601:1: C901 'LocalArrayBinaryOperation.__call__' is too complex (10)
distarray/plotting/plotting.py:68:1: C901 'plot_array_distribution' is too complex (15)
```

The `six` stuff isn't our problem. But the other functions definitely _look_ too complex.

This is low priority but I thought it was interesting. This might be a good issue for someone to tackle if they want to become acquainted with the codebase.