Add SurvivalAnalysisMixin as base class of ExtraSurvivalTrees

[sebp]

Checklist

• [x] closes #487
• [x] pytest passes
• [x] tests are included
• [x] code is well formatted
• [x] documentation renders correctly

What does this implement/fix? Explain your changes Adds SurvivalAnalysisMixin as base class of ExtraSurvivalTrees.

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.03%	:white_check_mark: 100.00%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (fa859b1e149fbe37e0b24160aeb420e746b337dd) | 3521 | 3490 | 99.12% | | | Head commit (d59f082355ae270a9c0b99fb47bc5b0bdcbed943) | 3521 (+0) | 3491 (+1) | 99.15% (**+0.03%**) | **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 (#488) | 7 | 7 | **100.00%** | **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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[codecov[bot]]

Codecov Report

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

Project coverage is 98.21%. Comparing base (fa859b1) to head (d59f082).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #488 +/- ## ========================================== + Coverage 98.15% 98.21% +0.05% ========================================== Files 37 37 Lines 3521 3521 Branches 464 464 ========================================== + Hits 3456 3458 +2 + Misses 31 30 -1 + Partials 34 33 -1 ```

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