wmean_1D copied into Cython version.
Cleanning core1D.
Reordering tests @ test_wmean_1D.
Fixing wmean_2D and serial for MaskedArrays & interp.
Improving and fixing test_wmean_2D.
Temprary solution for Cython version of wmean_2D.

It's a dummy function, that actually calls the Python version.
PEP8.