Add epidemic_peak() function

[pratikunterwegs]

This PR includes implementing epidemic_peak() originally taken from #152, and fixes #127. This was easier than rebasing the original branch on main.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 8bb4fb0 is merged into main:

• ✔️default_ode: 10.8ms -> 10.7ms [-1.63%, +0.68%]
• ✔️default_ode_interventions: 97.5ms -> 97.7ms [-2.16%, +2.45%]
• ✔️default_ode_param_vec: 830ms -> 829ms [-0.79%, +0.72%]
• ✔️default_ode_paramvec_intervs: 6.21s -> 6.21s [-0.64%, +0.64%]
• ✔️ebola: 553ms -> 555ms [-0.42%, +1.25%]
  Further explanation regarding interpretation and methodology can be found in the documentation.