Gave Group.hpp operators for comparing against other Groups
Split Groups/util.{cpp,hpp} into Util.hpp (for utilities that should be
accessible to the user of the library) and internals.{cpp,hpp} (for utilities
that shouldn't)
Slightly edited internals.hpp and tried to implement rel_ops-like comparison
operator templates that use the class's `cmp` method, if any (It didn't work)