fgemm with ModeTrait == GenericTag fails to compile

[ClementPernet]

There is some implicit assumption in fflas_fgemm.inl:412 (routine fgemm, DynamicPeeling and DynamicPeeling2) that the helper has Cmin, Cmax, Outmin, Outmax, etc. which is not the case with generic helpers.

[ClementPernet]

This is actually a duplicate of #106