Data id to str

2 small changes:
 - When asking for the filepaths to the data objects, instead of returning the filepath and the filepath type id, returns the filepath and the string of the filepath (this is required to get independence of Qiita-ware and the actual ids of the database)
 - The raw_data_id column on the database was not primary key, so I make it a primary key now.

Should be easy/fast to review!