More operators are numpy expr
Specialisation for numpy_expr
Do not enable "function on 1 elts" for numpy array.
Avoid argmin, argmax evaluation.

* Test on argmin((a + 5) * 2) and array of 100.000.000 elts
* old implem: 803 ms
* new implem: 582 ms
* CPython : 776 ms
* Same on 2D array:
* old implem: 790 ms
* new implem: 536 ms
* CPython : 783 ms
avoid temporary allocation for array_equiv

* With a = numpy.array([range(1000) for i in xrange(1000)],
* dtype=float); b = numpy.arange(1000, dtype=float).reshape(1000) and
* use of Expression template:
* old version : 71.6 msec
* new version : 68.8 msec
* CPython : 74.9 msec