Provide `CSET_CASE_DATES` and `CSET_TRIAL_PERIOD` in include files

### What problem does your feature request solve?

New information (CSET_CASE_DATES and CSET_TRIAL_PERIOD) needs to be made available to the METPlus side of the workflow.

### Describe the solution you'd like

At the moment I suggest we copy the relevant if...then...else structure to the include files (I am happy to do it as a separate issue), and we leave a more elegant solution for the integration of METPlus with the rest of the workflow.

### Describe alternatives you've considered

> [!NOTE]
> This issue is based on @JorgeBornemann's comment https://github.com/MetOffice/CSET/pull/765#discussion_r1807770959

Start by locating the include files used by the METPlus side of the workflow and trace where CSET_CASE_DATES and CSET_TRIAL_PERIOD are currently defined. Review the referenced discussion in pull request 765 to identify the relevant if/then/else structure, then verify that both values are available to METPlus without changing the broader integration design.

Written by the indexing model from the issue text.