Add 'day of week' effect as argument to Count observation process constructor (#718)

* add day of week effect code and tests

* adding tutorial

* more unit tests

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* changes per code review

* changes per code reivew; more unit tests

* changes per code review

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: cdc-mitzimorris

docs/tutorials/.pages

@@ -5,4 +5,5 @@ nav:
   - observation_processes_measurements.md
   - latent_hierarchical_infections.md
   - right_truncation.md
+  - day_of_week_effects.md
   - periodic_effects.md