Add support for array of complex numbers

Emulates real and imag attribute using a numpy_gexpr. Currently only
supports read mode.
Add error message when using an invalid method
Unfold range as loop parameter if unrolling failed

Otherwise we end up with very long literals where a simple for loop is
much more efficient.
Merge pull request #314 from serge-sans-paille/fix-complex-numpy

Fix complex numpy
Merge pull request #316 from serge-sans-paille/range-unfolding

Unfold range as loop parameter if unrolling failed
More operators are numpy expr
Specialisation for numpy_expr
Do not enable "function on 1 elts" for numpy array.
Avoid argmin 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