fix(algebra/group): workaround for #1871
fix(algebra/group): workaround for #1871

Currently, user attributes are not stored in `olean` files, which leads to segfault issues when referencing them using `user_attribute.get_param`. To work around this, we duplicate the stored data in an extra `def`, which *is* stored in the `olean` file.
leanprover-community/mathlib,leanprover-community/mathlib