Addition of Not available entry in controlled vocabulary

[ferrighi]

These are the main changes:

• doc: added "Not available" for dataset_production_status (updated iso mapping tables), iso_topic_category, operational_status and activity_type. Added "None" for vocabulary attribute.
• thesauri: added "Not available" for the above mentioned entries, with skos note for change.
• mmd.xsd: added values in list.
• dif-to-mmd: added "Not available" if Data_Set_Progress or ISO_Topic_Category are not provided
• ecmwf-iso-to-mmd: added "Not available" if gmd:status and gmd:MD_TopicCategoryCode are not provided. Fix vocabulary attribute from none to None
• iso-to-mmd: added "Not available" if gmd:status and gmd:MD_TopicCategoryCode are not provided. Fix vocabulary attribute from none to None and gcmd to GCMDSK.
• mmd-to-dif: do not map Data_Set_Progress and ISO_Topic_Category for "Not available" (in dif9 they are not mandatory).
• mmd-to-dif10: this was outdated (probably never revised). It has major changes.
• mmd-to-iso: skip mapping for "Not available" dataset_production_status and iso_topic_category, update vocabulary to GCMDSK and CFSTDN
• mmd-to-iso2: skip mapping for "Not available" dataset_production_status and iso_topic_category

[steingod]

This looks good to me. I couldn't see anything to change.