Abstract Iris lazy operations

[pp-mo]

Replace explicit use of biggus in cube.has_lazy_data() and creating / realizing lazy cube data.
Currently mostly tied to biggus Array and ArrayAdapter.

Acceptance Criteria :

• define common basis for cube lazy data handling, not explicitly referencing biggus or dask.
• refactor all the basic usages in the code to use the new abstract interface.

Something like ```iris._lazy_data`` as in #2318 -- but that needs reviewing.

[pp-mo]

Here is a PR : #2356