Make struct AnnualDate immutable

src/core/utils.jl

@@ -26,7 +26,7 @@ Stores a month and a day, and can be compared against normal dates.
 To save typing, a Tuple{Int64,Int64} is automatically converted to an
 AnnualDate, allowing this syntax: `nestingend::AnnualDate = (August, 15)`.
 """
-mutable struct AnnualDate
+struct AnnualDate
     month::Int64
     day::Int64