Fix broken links in documentation

[aaspeel]

• Fix broken hyperref to SOSTOOLS user's manual (References to chapters are still correct)
• Add hyperref to https://doi.org/10.1080/00029890.2001.11919774

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.85%. Comparing base (20236e4) to head (dbb5305).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #402   +/-   ##
=======================================
  Coverage   83.85%   83.85%           
=======================================
  Files          41       41           
  Lines        2477     2477           
=======================================
  Hits         2077     2077           
  Misses        400      400           

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[blegat]

Thanks Antoine ;)