Fix the usage of a metadata_file

[dcfidalgo]

Does not work at the moment, i think the ExamplePreparator needs to be updated.

[dcfidalgo]

Maybe add this as an extra parameter to the datasource yml config file:

forward:
  tokens: 
    source:
    - text
  gold_label:
    source: label
    mapping: label_mapping.txt

The source keyword should be omitted if the input of a forward parameter is just a string -> This should also be possible:

forward:
  tokens: text  # or tokens: [text] 
  gold_label:
    source: label
    mapping: label_mapping.txt

[dcfidalgo]

The usage of the metafile_data was reimplemented in #10
But we should unite metadata_file and values_mapping! Will open another issue for this.