Decouple metadata creation and indexing

[jean]

At the moment, a catalog metadata column is created for all "To be indexed" fields in Plomino.

Often we only want to query, but don't need the query results to carry all the metadata.

Make creation of metadata columns optional.

[ebrehault]

well, it would add an extra complexity in usage and I am not sure it makes a big difference regarding performances

what does impact the performances are indexes, not metadata
plus, if I remember correctly, in ZCatalog, when an attributes is indexed, the stored values are re-used for metadata (but i am not sure)