q_taylor_vector should be left as optional and have a default value

[moralapablo]

The use of an specific q_taylor_vector has become irrelevant in most cases as we enforce a lower polynomial order by using forced_Q_max. Then, the taylor vector can be set to be 8 at all non linear activation functions as it is the highest taylor expansion supported currently.

Still it would be useful to be able to define a custom one of needed.