Pandas should accept ISO 8601 duration
Hello,

ISO 8601 defines duration.
https://en.wikipedia.org/wiki/ISO_8601#Durations

see also usage in XML Schema http://www.w3.org/TR/xmlschema-2/#d0e11648

Duration can't be defined as `timedelta` (or `timedelta64`) because
- months don't always have 30 days 
- years don't always have 365 days

It will be nice if Pandas could accept ISO 8601 duration.

We should probably use `pd.tseries.offsets.DateOffset` like

```
pd.tseries.offsets.DateOffset(years=2, months=6, days=1)
```

Kind regards
