fix: fixes #491

[ThomasPiellard]

Description

Adds a method GenFrStar() in domain.go to get a generator of Fr* (wrongly named "generator of largest two adic subgroup" before").

Fixes # 491

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ivokub]

Looks good, but I'm not sure about the method name, I think it deviates from out current method naming pattern a bit.