row major xtensor assignment is so slow about xtensor

[spectre-ns]

I'm pretty sure this is slow because the column major xtensor will be strided in memory between elements. This will always be slow. I haven't run the assignment loop in this case but it could very well be using the stepper assignment method which won't be improved with openmp.

[tdegeus]

Could we quantify the performance by writing an explicit assignment in a loop (making use of the strides)?