Frailty survival model (#580)

* working frailty model and plots

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* adding colormap plots

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* adding aft regression models

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* adding more explicit comparison between lifelines, scipy and my estimates

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* extracted correct loglogistic fit removed lifelines models

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* adding some write-up

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* adding more write up and discussion

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* rearranging and adding more description

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* more write up and plot histograms

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* using a gamma prior on the frailty terms

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* running pre-commit checks

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* updated with pre-commit checks

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* added some references and write up

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* added discussion of acceleration factors

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* added conclusion and reference

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* added stratified hazard approach to final model

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* addressing Ben's comments

Signed-off-by: Nathaniel <NathanielF@users.noreply.github.com>

* adding fix for inline variable statements

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* adding fix for occurrence typo

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* correcting publication date and tidying

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* more tidying

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* more tidying and change final plot

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* added note about analogy with sorites paradox

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* addressing Ben's comments

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Author: NathanielF