Provide CSET_CASE_DATES and CSET_TRIAL_PERIOD in include files

[jfrost-mo]

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 #765 (comment)