Lazy data for GRIB.

Options:
 1. Continue to skip the test. :-1:
 2. Revert to testing the git repo.
 3. Enhance the testing command line so we can tell it where to find the git repo:
  - `python -m iris.tests.runner --example-tests --repo=...`
 4. *Any more?*

@QuLogic ... any thoughts?