mathlib3

9b6eef6e - feat(number_theory/modular_forms/congruence_subgroups): Add definition of congruence subgroups. (#15159)

feat(number_theory/modular_forms/congruence_subgroups): Add definition of congruence subgroups. (#15159) This contains the definition of a congruence subgroup of `SL(2,Z)` and defines Gamma1, Gamma0 and full level subgroups. It also contains some basic results about them.