Add SurvivalAnalysisMixin as base class of ExtraSurvivalTrees

[sebp]

Checklist

• closes ExtraSurvivalTrees missing score method since 0.23 #487
• pytest passes
• tests are included
• code is well formatted
• documentation renders correctly

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

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.03%	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (fa859b1)	3521	3490	99.12%
Head commit (d59f082)	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: <coverage of head commit> - <coverage of common ancestor commit>

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: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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