Check if all argsort use mergesort

[dachengx]

We must ensure that the numpy.argsort gives deterministic results, as well as in https://github.com/XENONnT/straxen.

This issue is closely related to XENONnT/straxen#1176.

[yuema137]

I checked the source code of numpy and confirmed that argsort is a more fundamental method while sort is a higher-level API based on it. The algorithms are defined in cpp files like https://github.com/numpy/numpy/blob/main/numpy/_core/src/npysort/mergesort.cpp

With the mergesort method, the relative order is preserved:
https://github.com/numpy/numpy/blob/020e78cc3a4c7283bbe83290f0be5de3bc842f07/numpy/_core/src/npysort/mergesort.cpp#L67

So as long as we use mergesort I think we are good here.

[dachengx]

So as long as we use mergesort I think we are good here.

Yes! So would you please check whether we always use mergesort?