Added mechanism to slice any lazy datasets contained in metadata. These
need to annotated at the class level by Sliceable. See the sliceable
test metadata class for an example
Update to tests, using model with azimuthal and changing mask to import
Tidied up sliceable class and removed debugging output
Updated models
Defaulted region.
Fixed buggy stop values in lazy datasets and added unit tests
Simplified and made more robust sliceable annotation by making it a
field annotation rather than a class one