array/dot : bug fix in strided case

- the general case (no blas call, vector with strides) was not correct.

- bug was introduced in the workaround blas bug of Apple
with dot, dotc for complex numbers.

- NB :  the code is currently written for a object with ImmutableVector
  concept but also with data_start (i.e. some real data).
 It won't work for a simple ImmutableVector.
 Generic code written in comment. Is it really slower ?
-> to be checked.