Adding double-dot functionality to defSurv

[kgoldfeld]

Addressing Issue #192

[github-actions[bot]]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 95c2a001261a68a06f38f5a9b2ac7fd691437885 is merged into main:

•   :ballot_box_with_check:define_data: 34.9ms -> 33.9ms [-8.71%, +3.07%]
•   :ballot_box_with_check:dist_beta: 856ms -> 867ms [-0.69%, +3.19%]
•   :ballot_box_with_check:dist_binary: 541ms -> 544ms [-1.77%, +2.72%]
•   :ballot_box_with_check:dist_binomial: 547ms -> 544ms [-5.11%, +3.95%]
•   :ballot_box_with_check:dist_categorical: 1.27s -> 1.25s [-4.99%, +2.07%]
•   :ballot_box_with_check:dist_exponential: 280ms -> 282ms [-2.03%, +3.92%]
•   :ballot_box_with_check:dist_gamma: 554ms -> 553ms [-4.2%, +4%]
•   :ballot_box_with_check:dist_mixture: 1.22s -> 1.23s [-1.98%, +3.84%]
•   :ballot_box_with_check:dist_normal: 555ms -> 545ms [-4.19%, +0.61%]
•   :ballot_box_with_check:gen_all_dists: 724ms -> 725ms [-2.7%, +2.93%] Further explanation regarding interpretation and methodology can be found in the documentation.