Add to_array, to_dataset

```
def to_dataset(self, dim) -> Collection:
	ds = xr.concat(self.values(), dim=dim)
	ds[dim] = self.keys()
	return ds
```

```
def to_array(self, dim) -> Collection:
	# check that all datasets only contain one array.
	da =  xr.concat(self.values(), dim=dim)
	da[dim] = self.keys()
	return da
```