Resolve "Add datatype of data / given answers to metadata"

We agreed upon the following:

• Enrich metadata yaml file with key datatype.
• Extend class Question by a property datatype, a method given_answers_according_to_type and change method as_series to regard the datatype of a series.
• Change class Answer to introduce the notion of "raw data", hold any kind of datatype in property data, add property datatype and question ID.
• Adapt module metadata to parse datatype from metadata yaml file.

Closes #29 (closed)