add a month for low frequency process

[AlexisEidelman]

For a annual frequent process (say marriage), the process is simulated every year during the same month.
That can produce an irrelevant seasonality. We could draw a random month of event in such case.

Won't be a piece of cake:

• the process is done during month 1 then the result must be saved somewhere until the month k selected.
  That's not obvious because of the internal structure of the simulation keeping only last period in direct memory.
  That's not the biggest issue
  • How can we deal with processes order. If divorce is run after marriage, just married couple are allowed to divorce but if the random month event for marriage is unfortunately after the divorce month, a bug is going to be generated.