Modified linear() to handle non-scalar length one coords.

I have a  PP file containing numerous phenomena many of which have a hybrid height coord. The file contains a single surface_altitude i.e. orography field. Some of the horizontal grids of the different phenomena are slightly offset. When I attempt to load this file I get:
```
ValueError: Cannot linearly interpolate a coordinate 'longitude' with one point.
```
It seems that rules.py is attempting to regrid the surface altitude but as the grid is shaped (14, 1) it is failing in ```linear()```as it cannot handle length one coordinates.

This PR modifies ```iris.analysis.interpolate.linear``` so that when one attempts to regrid a cube along coordinate associated with a length one dimension it extrapolates assuming a zero gradient. In other words, it assumes the data does not vary along that coordinate. I'm not suggesting this is the best idea in the world, but it allows me to load my data.