Extract schema from xarray objects

One way users may want to generate schemas is from xarray objects directly. We could add a utility here that supports this workflow.

This could be a function
```python
schema = create_schema(ds)
```

Or a class method:
```python
schema = DatasetSchema.from_dataset(ds)
```

Or as an Xarray accessor extension:

```python
schema = ds.schema.create()
```
