using Hecke's Small Groups without any extra relations for single cyclic groups

[Fe-r-oz]

This PR introduces Hecke.small_group for 'abelian' 2BGA codes for single cyclic groups. Hecke.small groups are limited, they can only be used single cyclic groups.

• The code is properly formatted and commented.
• Substantial new functionality is documented within the docs.
• All new functionality is tested.
• All of the automated tests on github pass.
• We recently started enforcing formatting checks. If formatting issues are reported in the new code you have written, please correct them. There will be plenty of old code that is flagged as we are slowly transitioning to enforced formatting. Please do not worry about or address older formatting issues -- keep your PR just focused on your planned contribution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.53%. Comparing base (f3eb7cd) to head (cba74cb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
+ Coverage   82.42%   82.53%   +0.10%     
==========================================
  Files          70       70              
  Lines        4580     4580              
==========================================
+ Hits         3775     3780       +5     
+ Misses        805      800       -5     

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