Converts a date from the French Republican Calendar to a Julian Day Count
frenchtojd ( int $month , int $day , int $year ) : int
The month as a number from 1 (Vendémiaire) to 13 (for the period of 5-6 days at the end of each year)
The day as a number from 1 to 30
The year as a number between 1 and 14
The julian day for the given french revolution date as an integer.