Allow ArrayHandle to have a runtime selectable number of buffers

Previously, the number of buffers held by an ArrayHandle had to be determined statically at compile time by the storage. Most of the time this is fine. However, there are some exceptions where the number of buffers need to be selected at runtime. For example, the ArrayHandleRecombineVec does not specify the number of components it uses, and it needed a hack where it stored buffers in the metadata of another buffer, which is bad.

This change allows the number of buffers to vary at runtime (at least at construction). The buffers were already managed in a std::vector. It now no longer forces the vector to be a specific size. GetNumberOfBuffers was removed from the Storage. Instead, if the number of buffers was not specified at construction, an allocation of size 0 is done to create default buffers.

The biggest change is to the interface of the storage object methods, which now take std::vector instead of pointers to Buffer objects. This adds a little hastle in having to copy subsets of this vector when a storage object has multiple sub-arrays. But it does simplify some of the templating.