feat(topology/algebra/group): added (right/left)_coset_(open/closed) (#11876)
feat(topology/algebra/group): added (right/left)_coset_(open/closed) (#11876)

Added lemmas saying that, in a topological group, cosets of an open (resp. closed) set are open (resp. closed). 





leanprover-community/mathlib,leanprover-community/mathlib