array:blas Fix dot to avoid blas 1 with complex

- On some machines, the zdotc, zdotu blas 1 are buggy
e.g. on OS X, but the pb seems to appear also on some linux.
- So for complex number, use the generic version of dot
- Also I wrote a generic version for dot, for 2 objects
modelling ImmutableVector concept.

	modified:   triqs/arrays/blas_lapack/dot.hpp