mathlib3

ef997baa - feat(representation_theory/group_cohomology/basic): add standard definition of group cohomology (#18341)

feat(representation_theory/group_cohomology/basic): add standard definition of group cohomology (#18341) We define the complex of inhomogeneous cochains, define group cohomology to be its cohomology, and prove this is isomorphic to the appropriate Ext groups. Co-authored-by: Joël Riou <joel.riou@universite-paris-saclay.fr> Co-authored-by: Amelia Livingston <al3717@ic.ac.uk>