Enhance _iter_comparable loop

[Vincent-Maladiere]

Checklist

• [x] pytest passes
• [x] code is well formatted

What does this implement/fix? Explain your changes Hello @sebp!

The sksurv.metrics._iter_comparable function repeatedly creates a mask using np.zero, but this mask doesn't change for different elements j of the loop. Mallocs are computationally expensive, so this short PR proposes to reduce the number of calls to np.zero by creating the mask before the for-loop.

This will help make the c_index functions more efficient. WDYT?

cc @ogrisel

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.15%. Comparing base (03566f6) to head (2de9df5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #465 +/- ## ======================================= Coverage 98.15% 98.15% ======================================= Files 37 37 Lines 3526 3526 Branches 539 539 ======================================= Hits 3461 3461 Misses 31 31 Partials 34 34 ```

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Coverage summary from Codacy

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Coverage variation	Diff coverage
:white_check_mark: +0.00%	:white_check_mark: ∅

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (03566f672b6075940a7d8722700c144412bf6d31) | 3526 | 3495 | 99.12% | | | Head commit (03566f672b6075940a7d8722700c144412bf6d31) | 3526 (+0) | 3495 (+0) | 99.12% (**+0.00%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#465) | 0 | 0 | **∅ (not applicable)** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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[sebp]

Thanks a lot for the PR.

Sorry for initially messing up your commit, but it's merged with commit ecd6dadbbaf217cc1cf8974f5b904b71d727c67d now.

[Vincent-Maladiere]

Awesome, thanks!