move schema out of data in an own object
rename identifieres from name to key and add name as custom lable
update README for schema and identifier naming
Merge branch 'feature/attributes' into develop

Introducing schema as an own object outside of the dataset.
To avoid confusion and to allow specific naming of columns all identifiers
are now call 'key'. The 'name' attibute can therefor be used for optional
descriptive naming.