Fix typo in julday_mod.F which affects simulations spanning centuries.

This issue is described in the Github issue:
   #48

The fix is to simply change this line of code:
   X1 = DBLE( YYYY ) / 100.0d0

to:
   X1 = DBLE( YEAR1 ) / 100.0d0

This commit passes a geosfp_4x5_benchmark difference test.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

GeosUtil/julday_mod.F

@@ -107,7 +107,7 @@ FUNCTION JULDAY( YYYY, MM, DD ) RESULT( JULIANDAY )
       ENDIF
 
       ! Compute the "A" term. 
-      X1 = DBLE( YYYY ) / 100.0d0
+      X1 = DBLE( YEAR1 ) / 100.0d0
       A  = MINT( X1 )
 
       ! The Gregorian calendar begins on 10 October 1582