Deduplicate 2-cohomology code

[stertooy]

Split off from #169. This doesn't contain any of the bugfixes.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.25%. Comparing base (78e5417) to head (7c12a92).

Files with missing lines	Patch %	Lines
gap/cohom/twocohom.gi	72.72%	3 Missing ⚠️
gap/cohom/intcohom.gi	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   61.71%   62.25%   +0.54%     
==========================================
  Files          87       87              
  Lines       13020    12909     -111     
==========================================
+ Hits         8035     8037       +2     
+ Misses       4985     4872     -113     

Files with missing lines	Coverage Δ
gap/cohom/intcohom.gi	10.71% <0.00%> (+8.58%)	⬆️
gap/cohom/twocohom.gi	74.41% <72.72%> (+0.17%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.