Index pfc_calendar field should inherit from Calendar

[imanuelcostigan]

Expected behaviour

library(fmdates)
library(fmbasics)
audbbsw <- AUDBBSW(months(3))
is.Calendar(audbbsw$pfc_calendar)
#> [1] TRUE

Actual behaviour

library(fmdates)
library(fmbasics)
audbbsw <- AUDBBSW(months(3))
is.Calendar(audbbsw$pfc_calendar)
#> [1] FALSE

This means that methods like to_maturity() do not adjust dates correctly. Calls from these methods to is_good() will return TRUE in all cases as it will use the default is_good() method