Replace validate_epiparameter() with assert_epiparameter()

[joshwlambert]

Breaking change

This PR addresses #361 by replacing the validate_epiparameter() function with assert_epiparameter(), and adds the test_epiparameter() function. The assert_epiparameter() and test_epiparameter() have the same logic but differ in their output and side-effects. assert_*() errors when the object is not an <epiparameter> whereas test_*() will return FALSE. This follows the {checkmate} naming convention and is a development following #364.

Functions that previously called validate_epiparameter() now call assert_epiparameter().

A bullet point is also added to the design_principles.Rmd vignette to give context to the development and naming of the class helper functions and methods.

[joshwlambert]

The test-coverage GHA is failing, but the tests pass on the R-CMD-check across all OSs, so I'll merge this PR and see if the tests continue to fail.