Add mean to std.algorithm

[JackStouffer]

Split from #3592

I will propose the same question I had in the other PR

I also want to add various other statistical methods to Phobos, such as standard deviation, which fit in std.algorithm, and things such as rolling mean, which don't fit into std.algorithm. I was thinking that something like a rolling mean would best fit in std.numeric, but, would it be best if all of the statistical methods were in the same module, or is it ok to have them separate?

Also fixes issue 14034

[burner]

as this function is not used directly in your tests I suppose it should be private and named meanImpl

[JackStouffer]

@burner See https://github.com/D-Programming-Language/phobos/pull/3679/files#diff-0d6463fc6f41c5fb774300832e3135f5R4395

[burner]

my bad

[9il]

Hi @JackStouffer!
I would like to see more PRs like that :-)
Few notes:

1. This and any other statistical PRs should depend on [new concept under development] Python's fsum and others #2991
2. We need to discuss module names first. A submodule for std.numeric is more suitable, thought. We should save std.algorithm.iteration as general purpose module without serious math payload.
3. Please ping @John-Colvin and @9il (me) when you create math PRs.

[JackStouffer]

This and any other statistical PRs should depend on #2991

Ok, I'm not quite sure how to make a branch off of a PR though.

We need to discuss module names first. A submodule for std.numeric is more suitable, though

std.numeric.stats perhaps? If that's the case, then your geometricMean function should be moved there as well.

We should save std.algorithm.iteration as general purpose module without serious math payload.

I agree. Following this, I believe the sum functions in std.algorithm.iteration should be deprecated aliases and move the actual functions in std.numeric then. That way all of the math code is in one place.

[9il]

Ok, I'm not quite sure how to make a branch off of a PR though.

You can fork my phobos repo and switch to the corresponding branch. Please read last comments starting with #2991 (comment) . Summation API can be changed during October. You are welcome for API review.

std.numeric.stats perhaps? If that's the case, then your geometricMean function should be moved there as well.

std.numeric.stats sounds good. For first release it may be small DUB package and std.experimental.numeric.stats.

[DmitryOlshansky]

Glad to see this is converging on nice package names and goes in the right direction ;)

[JackStouffer]

https://github.com/9il/phobos/pull/3