Duality and the row bound

• coverDuality : CoverDuality (linear-programming duality between fractional packings and rows; it corresponds to Proposition 6.7 of the paper, which is Dual.exists_cover; file Cover.lean, helpers in Sets, Functional, Core)
• rowBoundPM_of_packing : PackingBound → RowBoundPM (the row bound, Theorem 6.8 of the paper, from the packing bound, Lemma 6.5; file Row.lean, helper in RowAux)