Custom Index

Currently we are shoving some extra struff into the the attrs metadata:
e.g.

```python
{'c': 1.0, 'z': 0.5002025531914894, 'y': 0.3603981534640209, 'x': 0.3603981534640209}
ome_translation :
{'c': 0.0, 'z': 0.0, 'y': 0.0, 'x': 0.0}
```

but this really belongs as part of the coord metadata. to properly encode it I think we should subclass an existing xarray index and also keep track of this stuff as part of that object.