Fix so that circular logic can handle scalars

Logic has been added to the netcdf loader (see #1184) to set the circular flag based on the bounds. A user has hit an issue loading data (saved by Iris) where the longitude is a scalar. The current implementation assumes that 1d points arrays have 2d bounds, but in the case of scalar cf variables, the `bounds_data`  array is 1d. This PR modifies the logic to be a little more specific in the cases it handles.

This targets upstream/master as the new logic is not in v1.7.x