ENH: Added basic experimental raster import

This PR adds experimental raster import capability to iris.

This only functions as a first step (due to a lack of real test data - thus being in experimental).

1. Coordinate system not yet interpreted.
2. Multiple raster band supported but not validated (issues a warning to indicate as such - should be validated before leaving experimental)
3. Rotation assumed to be north-up (again a lack of test data and documentation - warning issued if rotated)

These are then key future enhancements in addition to the following:
1. Constrained loading
2. Deferred loading

Note: Unittests do not give full covergage and are added here for providing a starting point when this leaves experimental.