899 new add gamma distribution and other parameters to state age function 1

[lenaploetzke]

Changes and Information

Please briefly list the changes (main added features, changed items, or corrected bugs) made:

• added the parameters location and scale to StateAgeFunction
• added three state age functions
  • added tests

If need be, add additional information and what the reviewer should look out for in particular:

-

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Closes #929.

[codecov[bot]]

Codecov Report

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

Project coverage is 96.42%. Comparing base (deefc27) to head (92d7a37). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1004 +/- ## ========================================== + Coverage 96.36% 96.42% +0.06% ========================================== Files 130 130 Lines 10209 10360 +151 ========================================== + Hits 9838 9990 +152 + Misses 371 370 -1 ```

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[annawendler]

Can you merge the main branch so that the get_mean method is already included here?

[lenaploetzke]

Nice work!

I was wondering if we should rename ExponentialDecay to ExponentialSurvivalFunction. This would be consistent with the other new SAFs and describe better what it is used for. I'm not sure if SmootherCosineSurvivalFunction is a good name as I don't know if there is a name for the associated distribution, but I guess we should at least mention that this is a survival function as well in the documentation.

I like this idea and renamed ExponentialDecay. I also agree, that the name SmootherCosineSurvivalFunction is not appropriate, but put in the docu.