mathlib3

d1338749 - feat(representation_theory/basic): basics of group representation theory (#11207)

feat(representation_theory/basic): basics of group representation theory (#11207) Some basic lemmas about group representations and some theory regarding the subspace of fixed points of a representation. Co-authored-by: antoinelab01 <66086247+antoinelab01@users.noreply.github.com> Co-authored-by: Eric Wieser <wieser.eric@gmail.com>