Fix Earns to always returns a list, as documented (and other improvements)

[hulpke]

Recent improvements:

• The Knuth-Bendix uses a DAG for identifying which rule could apply at a given position. This cleans up other ad-hoc improvements, resulting in shorter and cleaner code
• The requested fix for Earns to agree with its definition.
• SubgroupConditionAbove works for arbitrary groups, not just permutation groups
• LowLayerSubgroups uses orbit in favor of pairwise conjugacy, if this is a plausible approach.
• Info statement in Dixon-Schneider prints more compact information.
• Clean up code duplication in compuattion of rewriting system for simple groups

Text for release notes

As promised in the manual, Earns now always returns a list.

[wilfwilson]

The fix for Earns is not complete. An empty list is now given instead of fail (which is good), but the other case still returns an actual group, rather than a list containing one group (which is what the documentation suggests).

gap> Earns(PrimitiveGroup(8, 3));
Group([ (1,7)(2,8)(3,5)(4,6), (1,3)(2,4)(5,7)(6,8), (1,2)(3,4)(5,6)(7,8) ])

[hulpke]

Earns(PrimitiveGroup(8, 3));

Thank you @wilfwilson -- did not realize that was part of the issue. Will fix.