Consider renaming `mean` to `arithmetic_mean`.

[n3vu0r]

Some functions like mean are implemented in ndarray (here) and ndarray-stats (here). When using SummaryStatisticsExt, the function mean exists two times, which is not a problem but confusing as one implementation returns Option and the other Result and it firstly seems to mismatch with the documentation.

Is it intended to have some implementations duplicated along the two crates on the long run? If so, ndarray-stats could be more specific by renaming mean to arithmetic_mean (and its axis/weighted variants) being consistent with geometric_mean and harmonic_mean. Feel free to close the issue if you prefer a simple mean.

[bluss]

In the long run, ndarray-stats should take over these methods from ndarray IMO.