break up create_clean_reported_cases()

[sbfnk]

Description

This PR closes #346 by breaking up the create_clean_reported_cases() function into its individual steps that are now separate functions that can be applied at preprocessing stage. After a deprecation cycle this function will be removed.

Doing so will facilitate addressing #640 via the new add_horizon() function that allows users to specify a forecast interval (internal for now, could later be exposed or accessed via forecast_opts() or the like when addressing #867).

It will also facilitate addressing #451 as data passed to functions can be used for plotting as preprocessing happens before calling estimate_infections() and the like.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• I have added a news item linked to this PR.

After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.

[seabbs]

I think there is an issue with the message formatting in the depreciation note.

[jamesmbaazam]

Might be a task for a separate issue but maybe we should showcase these functions in the examples of the exported functions.

[sbfnk]

Passing tests now so just needs someone to approve to enter the merge queue @seabbs @jamesmbaazam