Replace computing with delaying in the bilinear resampling

[mraspaud]

I'm not sure I understand why this needs to be computed and would prefer if things didn't have to be, but to get this merged I am ok with it.

Originally posted by @djhoese in #212

[pnuu]

The indices need to be computed for the slicing to work. With dask arrays the slicing result would have a shape of (y_slice.size, x_slice.size) when it should be (y_slice.size, ). The {y,x}_slize arrays are the same size as the target area. Also, the sliced data is still a dask array, so the slicing in self._slice_data()._slicer() is a lazy operation.