Renaming variables throughout for clarity

[damonbayer]

Closes https://github.com/CDCgov/multisignal-epi-inference/issues/156.

No changes to any of the model workings. Just renaming variables to make things easier to follow.

For example:

• objects named x_dist that are actually random variables are renamed x_rv
• object with vague names like latent are renamed to be more specific like latent_infections or latent_hosp_admissions
• predicted as a descriptor is replaced by observed or sampled where appropriate
• acronyms like ihr are replaced by more descriptive terms like infection_hosp_rate
• i0 is consistently formatted as I0

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.10145% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.75%. Comparing base (df0b4d5) to head (53d778c).

Files	Patch %	Lines
model/src/pyrenew/latent/hospitaladmissions.py	95.23%	1 Missing :warning:
model/src/pyrenew/model/admissionsmodel.py	95.23%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #179 +/- ## ========================================== - Coverage 94.77% 94.75% -0.02% ========================================== Files 39 39 Lines 842 839 -3 ========================================== - Hits 798 795 -3 Misses 44 44 ``` | [Flag](https://app.codecov.io/gh/CDCgov/multisignal-epi-inference/pull/179/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/multisignal-epi-inference/pull/179/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov) | `94.75% <97.10%> (-0.02%)` | :arrow_down: | 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.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.