Look up a registered rule
Examples
register_calendar_rule("demand", "sum")
get_calendar_rule("demand")
#> $rule
#> [1] "sum"
#>
get_calendar_rule("not_registered")
#> NULL
Look up a registered rule
register_calendar_rule("demand", "sum")
get_calendar_rule("demand")
#> $rule
#> [1] "sum"
#>
get_calendar_rule("not_registered")
#> NULL