mathlib

b9ad7333 - split(analysis/convex/combination): split off `analysis.convex.basic` (#9115)

split(analysis/convex/combination): split off `analysis.convex.basic` (#9115) This moves `finset.center_mass` into its own new file. About the copyright header, `finset.center_mass` comes from #1804, which was written by Yury in December 2019.