Adding support for self-referencing

[jannisborn]

First step toward #58

• paperscraper.citations.self_references allows to pass one or multiple DOIs and returns the author names and their self-citations. These are absolute counts per default, but setting relative=True returns percentages
• this can run asychronously
• changing default logger level from debug to info

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.26866% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.66%. Comparing base (0afbada) to head (cdbfcbe). Report is 1 commits behind head on citations.

Files with missing lines	Patch %	Lines
paperscraper/citations/core.py	90.19%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## citations #59 +/- ## ============================================= - Coverage 93.79% 91.66% -2.13% ============================================= Files 28 32 +4 Lines 967 1092 +125 ============================================= + Hits 907 1001 +94 - Misses 60 91 +31 ```

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