SL2.1: rename q to q0, explain q0=sqrt{q}

[fingolfin]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.92%. Comparing base (7ae6079) to head (dd391c4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   95.92%   95.92%           
=======================================
  Files          12       12           
  Lines         835      835           
=======================================
  Hits          801      801           
  Misses         34       34           

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