Add default observation rate to `compute_delay_ascertained_incidence`

[codecov[bot]]

Codecov Report

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

Project coverage is 93.72%. Comparing base (68c2b8d) to head (a0891b4). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #437 +/- ## ======================================= Coverage 93.72% 93.72% ======================================= Files 43 43 Lines 1052 1052 ======================================= Hits 986 986 Misses 66 66 ``` | [Flag](https://app.codecov.io/gh/CDCgov/PyRenew/pull/437/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/CDCgov/PyRenew/pull/437/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov) | `93.72% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov#carryforward-flags-in-the-pull-request-comment) to find out more.

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