DimCoord CML circular flag.

This PR is a simple change that introduces the missing cube metadata ```circular``` flag in the CML for a ```DimCoord```.

A simple change (see ```lib/iris/coords.py```), but naturally incredibly noisy, as it touches almost all CML test result files.

Once this PR is merged, PR https://github.com/SciTools/iris/pull/669 can be rebased.