Make scalar and packet paths consistent

Submitted by Gael Guennebaud @ggael

Assigned to Nobody

Link to original bugzilla bug (#1777)
Version: 3.4 (development)

Description

Currently many functions return slightly different results for coefficients evaluated using a SIMD path or a scalar path. This includes exp, log, logistic_function, etc.

Since the SIMD implementation of those functions can also be called on scalar inputs, we could easily solve this inconsistency by plugging the respective functor call to the generic SIMD path.

Shall we do that unconditionally or only if vectorization is enabled?

Blocking

#814 (closed)