Remove all uses of *.tag.test_value and replace with a new initial value framework #4567
Description
We need to set up a framework for setting and getting initial values for a model; one that doesn't rely on Aesara's test values.
In many cases, this may be as simple as renaming parts of the existing API (e.g. Model.test_point to Model.initial_point, Model.check_test_point to Model.check_initial_point, change the testval Distribution argument to initial_value, etc.), but, in some cases, we'll need to add new variables and logic.
Specifically, it seems like we'll need to store the initial values in the Model object and, in multiple places, obtain those values from a Model object.