Add support for early-stopping to GradientBoostingSurvivalAnalysis

[sebp]

Checklist

• closes n_iter_no_change in GradientBoostingSurvivalAnalysis #334
• pytest passes
• tests are included
• code is well formatted
• documentation renders correctly

What does this implement/fix? Explain your changes
Add support for early-stopping to GradientBoostingSurvivalAnalysis by setting n_iter_no_change.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (52695a8) 97.88% compared to head (f33600b) 97.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   97.88%   97.92%   +0.03%     
==========================================
  Files          37       37              
  Lines        3306     3319      +13     
  Branches      450      454       +4     
==========================================
+ Hits         3236     3250      +14     
  Misses         33       33              
+ Partials       37       36       -1     

Impacted Files	Coverage Δ
sksurv/ensemble/boosting.py	97.47% <100.00%> (+0.38%)	⬆️

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