Fixed const correctness of ArrayHandle

There are some methods in ArrayHandle that are not declared const but should be. One example is PrepareForOutput. The idea for not declaring them const was so that if you had a const ArrayHandle you should not be able to modify its contents. But this really doesn't work because you can easily (and accidentally) pass it by value and then suddenly a pointer to the same array is non-const.

The only way to really may an ArrayHandle const is to make the ValueType a const type. Otherwise, methods that potentially change the data should be declared const. (And if you actually try to change the data, e.g. through a portal, you will get an error there.) We should only declare a method non-const on ArrayHandle if it changes the immediate data structure (such as allocating or sizing the array).