Export of metadata subtly broken

[daxtens]

excel.py export

It is hard to distinguish between metadata that is supposed to be at top level, and metadata that is supposed to be a column. Doing a metadata search at top level will make everything a column. Attempting to then remove the top level metadata (current approach) will silently fail to export metadata common to parent and children. Correct approach is to get the union of the sets of metadata of all the children.