Process returns tuple with array equal to duration (patch to PR 123)

[gvegayon]

This addresses and should close #142. The PR does the following:

• Corrects all classes in pyrenew.process to return tuples with arrays of shape (duration,).
• Makes n_timepoints optional for models under pyrenew.model. Only required when observed data is not passed, the length of the time series is inferred from the observed data.
• Consequently, updates tests and tutorials, only using the n_timepoints argument when simulating data.
• It also adds additional assertions to the process testing to make sure the length of the output matches duration.

Notice this PR will be merged in to #123.

cc @damonbayer and @dylanhmorris

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 93.04%. Comparing base (b603eb5) to head (8ead934).

Files	Patch %	Lines
...odel/src/pyrenew/model/rtinfectionsrenewalmodel.py	37.50%	5 Missing :warning:
model/src/pyrenew/model/admissionsmodel.py	57.14%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 9-infection-feedback-in-rt #143 +/- ## ============================================================== - Coverage 93.86% 93.04% -0.83% ============================================================== Files 35 35 Lines 734 748 +14 ============================================================== + Hits 689 696 +7 - Misses 45 52 +7 ``` | [Flag](https://app.codecov.io/gh/CDCgov/multisignal-epi-inference/pull/143/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/143/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov) | `93.04% <52.94%> (-0.83%)` | :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.

[gvegayon]

Hold my horses, I realized this should be merged into #131 instead! Thank goodness we have git cherry-pick. I'm closing this.