Converts a date from the French Republican Calendar to a Julian Day Count.
These routines only convert dates in years 1 through 14 (Gregorian dates 22 September 1792 through 22 September 1806). This more than covers the period when the calendar was in use.
The month as a number from 1 (for 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.