Support using arrays with dynamic Vec-likes as output arrays

When you use an ArrayHandle as an output array in a worklet (for example, as a FieldOut), the fetch operation does not read values from the array during the Load. Instead, it just constructs a new object. This makes sense as an output array is expected to have garbage in it anyway.

This is a problem for some special arrays that contain Vec-like objects that are sized dynamically. For example, if you use an ArrayHandleGroupVecVariable, each entry is a dynamically sized Vec. The array is referenced by creating a special version of Vec that holds a reference to the array portal and an index. Components are retrieved and set by accessing the memory in the array portal. This allows us to have a dynamically sized Vec in the execution environment without having to allocate within the worklet.

The problem comes when we want to use one of these arrays with Vec-like objects for an output. The typical fetch fails because you cannot construct one of these Vec-like objects without an array portal to bind it to. In these cases, we need the fetch to create the Vec-like object by reading it from the array. Even though the data will be garbage, you get the necessary buffer into the array (and nothing more).

Previously, the problem was fixed by creating partial specializations of the Fetch for these ArrayHandles. This worked OK as long as you were using the array directly. However, the approach failed if the ArrayHandle was wrapped in another ArrayHandle (for example, if an ArrayHandleView was applied to an ArrayHandleGroupVecVariable).

To get around this problem and simplify things, the basic Fetch for direct output arrays is changed to handle all cases where the values in the ArrayHandle cannot be directly constructed. A compile-time check of the array's value type is checked with std::is_default_constructible. If it can be constructed, then the array is not accessed. If it cannot be constructed, then it grabs a value out of the array.