Download references

[wergstatt]

The major tasks will be to load references instead. Thus, ...

• [x] modify get_pap_info.py to include reference information and write those into a separate table.
• [x] ensure that #13 allows to include two columns in the automated historization.
• [ ] both columns of the references table should reference the UUID of the paper as foreign key, thus ensuring the "relevance" of the article.

[wergstatt]

part 3 will be accomplished via #15 and shifted to the according issue