pyon: Add xarray object encoding and decoding.

[ThorstenGroh]

Hey, thanks for the nice work here.

I would like to send xarray objects via RPC, but sipyco was missing a decoder/encoder for xarray objects (xarray.pydata.org/). Xarray DataArray and Dataset objects have a to_dict method, that can be easily used to transform the object to something serializable (one however needs to bypass the conversion of numpy objects to lists).

Let me know if this is within the interest of this package and how to proceed.

I added the necessary encode and decode methods and a unittest. Is there anything else to do?

Best regards
Thorsten

[sbourdeauducq]

We don't want to add a mandatory dependency on xarray.

There would be several ways to address this:

• handle ImportError and disable the corresponding functionality accordingly.
• add a with_xarray parameter and import xarray dynamically.
• a plug-in system, though PYON: add plugin support #22 is overly complicated. Simply extending the handlers using some dictionary or class passed to encode/decode would be better. Then the plugin could be integrated into sipyco.

[jordens]

#64 adds a plugin interface. feel free to revise this PR for optional xarray support.