Issue #688 - Reorganise `validate_forecast()`

[nikosbosse]

Description

This PR closes #688. This PR closes #689.
This PR closes #773.

In #688 and in particular in #753 we discussed the relationship between validate_forecast() and as_forecast() and decided on the following:

• as_forecast() converts from a non-forecast object to a forecast object, validates it and returns the object
• assert_forecast() re-validates an existing forecast object and returns invisble(NULL)
• validate_forecast() is like assert_forecast(), but returns the object so it can be used in a pipe.

This PR

• renames the current validate_forecast() to assert_forecast() and changed it to return invisble(NULL)
• introduces a silent argument to validate_forecast() (that effectively prevents one single message from being thrown)
• uses that new silent argument, e.g. when printing
• creates a new validate_forecast() function that calls assert_forecast() and returns the data
• has a change with a questionable relationship to this PR! <3
  • namely it renames validate_general() to assert_forecast_generic()
• updates the News file
• updates the manuscript
• does not have any new tests. Should it? I had a vague feeling it should, but I'm also a bit unclear what to test....

Further question: Should as_forecast() get a silent argument as well? We have quite a large number of suppressMessages() calls in our tests, which maybe is a hint...

[Describe the changes that you made in this pull request.]

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.60%. Comparing base (aa01cad) to head (573166d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #778 +/- ## ========================================== + Coverage 95.53% 95.60% +0.06% ========================================== Files 21 21 Lines 1567 1569 +2 ========================================== + Hits 1497 1500 +3 + Misses 70 69 -1 ```

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[seabbs]

Should as_forecast() get a silent argument as well? We have quite a large number of suppressMessages() calls in our tests, which maybe is a hint...

I think if we rename it to be verbose then yes

[nikosbosse]

Latest update:

• added missing importFrom for cli_warn()
• changed silent to verbose
• made sure those verbose statements actually get passed on to assert_forecast()
• added new unit tests, in particular making sure that we get a warning when there are different numbers of quantiles, but no warning when printing (as assert_forecast() is in silent mode)
• updated docs/namespace