Include CellMeasures and AncillaryVariables in repr_html

Raised as part of this [review comment](https://github.com/SciTools/iris/pull/3422#issuecomment-541785295) 

It appears the CellMeasure and, once #3422 is in, AncillaryVariables are not handled in the `iris.experimental.representation` content handling