Merge the occupancy information of assets into the taxonomy string

To allow for faster and easier processing in Rabotnik, all relevant information to assign assets to an entity should be in the taxonomy string (as described in the taxonomy documentation). This means, that the occupancy field in Asset should be merged into the Taxonomy.taxonomy_string field. The following steps need to be taken:

• Remove occupancy from Asset
• Adjust the writing and reading functions
• Adjust the importer and exporter
• Develop functions to split the taxonomy string into taxonomy without occupancy and occupancy

\rfc @tara @shinde @laurens