Small fix in resampling test.
Cleanup in resample_img.

Added tests, reorganized code in resample_img.
Optimized memory usage for resample_img()

The original resample_img used np.concatenate, which caused the memory usage
to double shortly before leaving the function. This is not the case anymore.

The execution time is slightly shorter (of the order of 5%).

Added an "order" keyword to resample_img.
"copy" kwarg in resample_img works in all cases

In some no-op cases, the "copy" keyword did nothing.
Added some tests cases. 100% test coverage for nisl.resampling.