Add is_pairwise function

[thofma]

I find this handy and wanted to be a good citizen by adding it here instead of Hecke.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.17%. Comparing base (3e14b81) to head (092ecbc).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2326   +/-   ##
=======================================
  Coverage   88.17%   88.17%           
=======================================
  Files         126      126           
  Lines       32073    32081    +8     
=======================================
+ Hits        28279    28287    +8     
  Misses       3794     3794           

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[fingolfin]

I like it. Should we also hook it up in the manual?

[thofma]

Done