transforms a number into a date
dat=num2date(num,f)
* num = a date in numerical presentation
* f = dates frequency (in constant format)
* dat = a date in string format
num2date(200,1) // returns "200a" num2date(200,4) // returns "49q4" num2date(200,52) // returns "3/11/03" num2date(200,26) // returns "7f26d18"