BUG: Extraction of scalar cubes

Fixes the issue with performing extraction on scalar cubes or cube lists containing scalar cubes.

**Approach taken:**  Utilise existing index based code (_ColumnIndexManager) by taking a copy of the original cube and reshaping it (ndim=1).  Taking this copy allows us to return the original scalar cube (this is consistent with the current behaviour of non scalar cubes where the constraint matches the entire cube).

**Note:**  The approach taken copies the scalar cube and also touches the data in reshaping it.  We don't have to make a copy if we are content with not returning the same cube object.