For easy calculation. Does not handle cprs with letters (interim cpr)
Examples
cpr_dob("231045-0637")
#> [1] "23-10-1945"
fsd<-c("2310450637", "010190-2000", "010115-4000",
"300450-1030","010150-4021")
cpr_dob(fsd)
#> [1] "23-10-1945" "01-01-1990" "01-01-2015" "30-04-1950" "01-01-1950"