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]

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 ⚠️

Additional details and impacted files

@@              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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