[csvwlib] properties at table level

[pvgenuchten]

in csvw you typically register some metadata elements at table level

it is also possible to describe multiple tables in a single csv

this relates to #232, suggesting to move the attributes element one level deeper

content-info:
   - featuretypes: 
      - name: trees
        url: https://www.wikidata.org/wiki/Q10884
        attributes:
           - name: length
           - name: type
     - name: plants
       url: https://www.wikidata.org/wiki/Q756
       attributes:
          - name: length
          - name: type