Deprecated ensure_array

This PR deprecates ```iris.util.ensure_array```. It is only used in a couple of places throughout the codebase and its use is redundant because the cube has a property setter for ```cube.data```. This work was down because of issues/inconsistencies when collapsing cubes down to scalar cubes. As a result it was also necessary to modify the ```iris.tests.IrisTests.assertMaskedArrayEqual``` to handle masked numpy 0d arrays.