Allow multiplication in duration input value

[lukaszachy]

To make it easier for adjust the multiplication happens last: No need to pay attention how adjust creates the value.

Only late I realized the input value for multiplication should accept float, so I will modify that later. For now: does this approach will be accepted? It is preparation for #1843 so "adjust" doesn't need special handling of duration key.

Pull Request Checklist

• [x] implement the feature
• [x] write the documentation
• [x] extend the test coverage
• [ ] mention the version
• [ ] include a release note