The Proposed Coordinates API

This PR will contain all work on the coordinates API. The API tests are in a separate PR, and regular tests pertaining to various classes in the coordinates framework will be introduced in this PR.
Comments are very much appreciated!
replace:
```Python
if 'D0' in kwargs:
    kwargs['distance'] = kwargs['D0'] - kwargs['zeta']
else:
    kwargs['distance'] = self.D0 - kwargs['zeta']
```

with

```python
D0 = kwargs.get('D0', self.0)
kwargs['distance'] = D0 - kwargs['zeta']
```