Subdev specs can now be iterated over in Python as in C++:
```python
specs = usrp.get_rx_subdev_spec(0)
for spec in specs:
print(spec.to_string())
```
This would previously fail, because Pybind11 did not know that the
subdev spec was iterable.
- Separating exposed Python data structures into logical sections
- Exposes all of the multi_usrp API
- Adds a layer of Python for documentation and adding helper methods
- Adds improvements and fixes to the MultiUSRP object
- Includes additional exposed data structures (like time_spec_t, etc.)
- Add code to release the Python GIL during long C++ calls
