accumulate for observed reports

[sbfnk]

Description

This PR closes #640. However, this revealed the complication that we don't currently have a way to set the frequency of forecasts (e.g. weekly etc). This could potentially be addressed by the suggestions in #346 (comment) and should be done first.

The suggestions in here should eventually be merged here as it makes the predictions consistent with data input.

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]

Is this still something that will progress or has it been replaced by the other similar work?

[sbfnk]

It has been superseded by #901 (and a subseqeunt PR to address #640).