mathlib3

74f1d619 - feat(analysis/convex/cone/proper): define proper cone (#18913)

feat(analysis/convex/cone/proper): define proper cone (#18913) Part of #16266 We define proper cones as nonempty, closed cones and define duals. Next todo: Prove Farkas' lemma from #19008 Co-authored-by: Apurva Nakade <apurvnakade@gmail.com@>