Load raw reduced grids from GRIB files.

Adds the ability to load reduced grids from GRIB files in raw form, i.e. without automatically interpolating to a regular grid. The change is made at a fairly low level in the API so one needs to do extra work to use it, but I think it is important that the ability to do this at all arrives first, and we can decide if/how it should be integrated into the load procedure at a later time.

Current usage:

```python
import iris
cube_generator = iris.fileformats.grib.load_cubes("reduced.grib",
                                                  auto_regularise=False)
cubes = iris.cube.CubeList(cube_generator).merge()
```

Pinging @PAGWatson since he asked for this feature, feel free to comment here.