Cleanup traits for detection of barett and SIMD modular reductions

[ClementPernet]

In fflas_freduce.h

• the trait support_fast_mod stands for "the field element" support Barret's modular reduction provided that the field is of Modular type (Modular or ModularBalanced)
• the trait support_simd_mod means the same with the additional information that some vectorization can be used for the reduction. These traits are mostly used in fscal, fscalin, freduce and faxpy,faxpyin. At some point the names of these traits should be clearer and uniformized with more general traits detecting Modular type fields (avoinding to have these traits depending on another trait (modular type fields).