Re-implement the `@xpersist` decorator

Something along the lines of 

```python
  import xarray as xr
  import xpersist as xp
  import numpy as np

  In [1]: @xp.persist(cache=..., key=...)
      ...: def func(scaleby):
      ...:     return xr.Dataset({'x': xr.DataArray(np.ones((50,))*scaleby)})

```