Bicubic interpolation fix and reference

See: https://github.com/scikit-image/scikit-image/issues/1035
This test fails now. It seems that the function `coord_map` is broken. The only mode, that returns correct results is `NEAREST`. Even `CONSTANT` returns different results right at the edges for NN interpolation.

I'd appreciate any help in investigating this further. I don't see the difference to Scipy... and I cannot figure out what would be wrong even with the simplest case, I described above.