Align Data Model with current implementation

[nichtich]

See example of current data model:

• make elements an object, so every element has a locally unique key
• label is an array. This should better be language map instead, for multilinguality. Multiple labels of the same language is only used on one special case: ["LIDO Record","Wrapper for an object record"], so it can likely be dropped to reduce complexity.
• paths vs. path - what's the best practice? Is there a difference between "path":"//foo|//bar" and "paths":["//foo","//bar"] for XPath?
• there are both elements and properties - is there a fundamental difference (not bound to XML) or can be combine both?
• what is the semantics of namespace?
• additional metadata, external to the data model file in config.json, should better be made part of a schema:
  • schema_file => schemas (repeatable) with URL/file and schema language?
  • can semantics of namespaces be defined abstract from concrete formats such as XML?
  • root_element_name, root_element_path, record_identifier_element_name, record_identifier_element_path is duplicated information. They can be replaced by plain string fields such as record and recordid pointing to the entry from elements

See corresponding issue in Constrainify for implementatio