Cells are not working (yet?)

The following code returns a segmentation fault, simply because SPICE cells are not supported:

``` python
startUTC = "25 FEB 2013 00:00:00"
endUTC   = "28 FEB 2013 00:00:00"

startET = utc2et(startUTC)
endET   = utc2et(endUTC)

c = wninsd(startET,endET)
```

At least we could implement a more graceful failure.
