Check that dimension ordering of iris cubes is consistent through a run of MIP Convert

In the exploration of #678 we found that iris can in some circumstances provide different coordinate ordering depending on whether the time axis has length 1.

We could add a check in to MIP convert to ensure that every iris cube passed through to be written to file by CMOR has a consistent set of dimensions for every chunk written.

Start by reading issue #678 and tracing the MIP Convert path where Iris cubes are passed to CMOR for writing. Identify how cube dimensions are currently handled across chunks, then add a consistency check and verify that inconsistent ordering is detected while consistent runs still write successfully.

Written by the indexing model from the issue text.