Correct dim coord behaviour for aggregated by.

This PR ensures that a cube containing a CF dimension coordinate that is being aggregated over, results in an aggregated cube with the CF dimension coordinate maintained, if possible i.e. the aggregation operation can cause a ```DimCoord``` to be demoted to an ```AuxCoord``` due to the aggregation sampling applied.

Previously, the CF dimension coordinate once aggregated was **always** grouped as an auxiliary coordinate, regardless of whether it maintained its ```DimCoord``` status.