Improve graph definition in flow.cylc to avoid duplication

[jfrost-mo]

Decidedly not an issue for this PR, but I wonder if I can structure this better to avoid the duplication of the graph definition. Something like:

{%  # Pseudocode.
if CSET_CYCLING_MODEL == "case_study":
    recurrences = ",".join(f"R1/{date}" for date in CSET_CASE_DATES)
else:
    recurrences = CSET_TRIAL_CYCLE_PERIOD
%}
    {{recurrences}} = """
    metplus_ascii2nc => metplus_point_stat
    """

I'll open a separate issue for this.

Originally posted by @jfrost-mo in #1906 (comment)

This both makes the graph more readable, and avoids the possibility of the two copies getting out of sync.