Better congruence lattices

[james-d-mitchell]

This PR properly integrates the code for better congruence lattices written in conjunction with @mariatsalakou and @marinaanagno.

[james-d-mitchell]

Just to mention that the test file: tst/extreme/cong.tst goes from taking 42,930ms before this PR to 1,877ms with this PR, which is a nearly 23 times speedup, and some evidence that the changes implemented do actually improve the performance

[james-d-mitchell]

@wilfwilson any idea why the github actions are failing with:

Run # optionally: clone specific package versions, in case we need to test
fatal: destination path 'digraphs' already exists and is not an empty directory.
Error: Process completed with exit code 128.

I don't think I changed anything related to this in this PR.

[wilfwilson]

I think this is gap-actions/setup-gap#38.

The script isn't successfully deleting an existing package.

Previously this was just annoying, because the installed package was in a directory called like digraphs-1.5.0 or something, which doesn't stop up us cloning into the digraphs directory.

But presumably Digraphs is just called "digraphs" in the package distribution, so it does already exist, and so we can't clone into it. So gap-actions/setup-gap#38 needs to be fixed.

[james-d-mitchell]

Thanks @wilfwilson!