StorageType is public in vtkm::cont::ArrayHandle, but some subclasses
re-declare it as private. Having it public provides a convenient way to
get the storage type of an ArrayHandle, otherwise it is quite verbose.
Addresses: #314
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.
