Allow datetime-like values for LinearInterpolator.

Allows you to write things like:
```python
cube.interpolate([('time', datetime(2014, 7, 4))], Linear())
```
Which, while a little bracket-heavy, is still a lot better than `this.that.num2date(the.other.date2num(......`

Closely related to #1231.
Any idea of what performance impact this has?