Fix value_type for numpy_expr

* value_type of : empty_like(ndarray<long, 2>) should be ndarray<long,1> instead of long
Add dtype and T attributs to numpy array
Fix shape for gslice

* case of gslice without all its dimension named in the slice
* ex: a.shape = (2,3,4)
    b = a[:,1]
    print b.shape
gslice are built from const array
WIP