Grothendieck groups by presentation

GrothendieckPresentation is generic in its generator and relation types — it mentions no category at all — and K₀Ab instantiates it at the short exact sequences of an abelian category.

Neither file uses anything triangulated, which is why they live here rather than under Triangulated/. The triangulated instantiation K₀, built on distinguished triangles, stays at Triangulated/GrothendieckGroup/ where it belongs.