Issue 34: rephrase integral as an ODE

[seabbs]

Description

This PR closes #34.

Under the hypothesis that more work has been put into the ODE solvers vs the integrator and that they may therefore be more stable, I recharacterised the integral as an ODE problem and solved it using the ode_rk45 solver (likely worth exploring other options as well as tolerances). This appears to give a 4-5 times speed up and on the test data has no stability issues (i.e no chains out of 600 failed).

I am very unclear as to exactly why apart from above logic about dev time spent.

As part of testing this I have made the simulated logic significantly harder (a mix of window lengths and observation times).

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• [x] I have read the contribution guidelines.
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] My code follows the established coding standards.
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• [x] I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov[bot]]

Codecov Report

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

Project coverage is 99.62%. Comparing base (c81f6dc) to head (2d62937). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #64 +/- ## ======================================= Coverage 99.62% 99.62% ======================================= Files 7 7 Lines 268 268 ======================================= Hits 267 267 Misses 1 1 ```

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