extending attributes to capture featuretypes and protocols

[pvgenuchten]

a draft pr, to indicate an idea to extend attributes with featuretypes in line with csvw and iso19110 (feature catalogue)

• keep current attributes for backward compatibility
• move attribute to dedicated type so it can be used in dimensions and attributes
• introduce array of featuretypes in contentinfo which itself have a attributes property
• on featuretype enable identifier/foreign key as defined in csvw

also merging dimension and attribute contents

for now only updated the mcf schema

[tomkralidis]

In summary:

• introduce content_info.featuretypes (1..n featuretype)
• each featuretype is an object with an attributes array
• +1 for backwards compat
• unify attributes/dimension definitions