feat(rust): Add min periods for correlation (#15458)

[fkemeth]

Add min_periods support to correlation (pearson, spearman and cov). See #15458.

[codecov]

Codecov Report

Attention: Patch coverage is 90.69767% with 8 lines in your changes missing coverage. Please review.

Project coverage is 80.73%. Comparing base (d13efa5) to head (8c83322).
Report is 4 commits behind head on main.

Files	Patch %	Lines
...s/polars-plan/src/dsl/function_expr/correlation.rs	81.25%	6 Missing ⚠️
crates/polars-ops/src/chunked_array/cov.rs	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16987      +/-   ##
==========================================
- Coverage   80.74%   80.73%   -0.01%     
==========================================
  Files        1475     1475              
  Lines      193389   193427      +38     
  Branches     2760     2760              
==========================================
+ Hits       156146   156164      +18     
- Misses      36733    36753      +20     
  Partials      510      510              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ritchie46]

Thank you @fkemeth, I think this looks pretty good. However, we should not return NaN if below min-periods. We use None on other places where we have a min-periods argument.

[fkemeth]

@ritchie46 thank you for the comment! I updated the PR accordingly. The functions return Null now if min_periods>n.
This entails that, afaik, the return type of cov and online_pearson_corr must be Option<f64> now, instead of f64.

I also updated the unit tests, which now check for

    assert not out1["pearson"][0]
    assert not out1["spearman"][0]

if min_periods is larger than the common overlap between the two vectors.

Can you have a look?