arrays : fix auto_assign for chains assignment.

A(i_)(om_) << ...
for A an array of gf was not working.
Modified the auto_assign of arrays to handle the case when the object
in the array is itself autoassigned.
Using the model of std::vector adapter for clef, which works.
Also fixed the gf for a little details (gf_impl is usually in the expression tree, not gf).