Implement high-level table and image SAMP routines

Once #1907 is merged, it would be worth writing simple functions that can take a filename, a ``Table``, or an ``NDData`` object and send them over SAMP (optionally to a specific client).

@taldcroft - one idea I had would be to add two methods to Table:

```
>>> t = Table(...)
>>> t.send_via_samp(...)
```

would make the table appear e.g. in TOPCAT or DS9, and:

```
>>> t.receive_from_samp(...)
```

would be a class method that would 'hang' until a table is received via SAMP. The same could be done for NDData.

cc @cdeil @keflavich