Reading H⁰ in a presented category, and in its Grothendieck group

What is left here after #1320 are the declarations that need something beyond the dg category: either a chosen ordinary category and the comparison equivalence of an Enhancement, or the Grothendieck group of the triangulated structure.

The intrinsic side -- the shift, the distinguished triangles, the functorial cone diagrams and the exactness of DGFunctor.h0 -- moved to its definition owner, Algebra/Homology/DGCategory/Pretriangulated/H0/, and nothing there imports this directory.

ConeFunctor and MorphismCone transport dg cones across a comparison, and carry that comparison's shift and exactness compatibilities as hypotheses. The four K0 modules read triangles and twists in K₀, and consume CategoryTheory/Triangulated/GrothendieckGroup/.