Use full Iris load chain

[pp-mo]

Iris loading is currently using the iris.fileformats.netcdf.load_cubes() module entry point.
This was not the original intention.
In fact, we added the "data" io loading scheme specifically to support loading from a dataset object, to support ncdata.
Using this will support full load chain, including new flexible combine processing, allowing e.g. concatenating between monthly data files