implement ranking method

the usual way to implement it in numpy is `a.argsort().argsort()` which would translate to `a.posargsort(axis).posargsort(axis)` in larray

for large 1D arrays, another technique is faster, but not sure it's worth it for now:
https://gist.github.com/naught101/14042d91a2d0f18a6ae4#file-python_rank_benchmarking-py-L17