Allow datetime.timedelta coordinates.

[akleeman]

This would allow you to have coordinates which are offsets from a time coordinates which comes in handy when dealing with forecast data where the 'time' coordinate might be the forecast run time and you then want a 'lead' coordinate which is an offset from the run time.

[shoyer]

In fact, I have a fix for this in #54. See utils.safe_cast_to_index.

[shoyer]

It would be useful, however, to have automatic encoding/decoding of such variables to NETCDF.

[shoyer]

For a full solution, it would be great if pandas included a TimeDeltaIndex so we don't have to use an index with dtype=object. Here is the pandas issue: pandas-dev/pandas#3009

[shoyer]

pandas.TimedeltaIndex will finally arrive in the next version of pandas. It would be nice to support it with properly encoding/decoding.