Previously, `ArrayHandleSOA` worked with any value type that supported
`VecTraits`. That means that `ArrayHandleSOA` worked with scalar types
like `Float32`. However, for scalar types, the behavior is essentially
the same as `ArrayHandleBasic`, but with lots of extra templating and
code generation.
Although there is nothing _wrong_ with allowing `ArrayHandleSOA` holding
a scalar, there is no real reason to either (other than likely template
convenience). Generally, there is nothing wrong with supporting it, but
if you want to support `ArrayHandleSOA` in places where types are not
known (e.g. `Field`), then templating tends to iterate over the cross of
all supported types with all supported storage. That means such code
will automatically generate a bunch of code for `ArrayHandleSOA` with
scalars even if there is no reason for those code paths.
So, we can just disable the use of `ArrayHandleSOA` with scalars to
allow us to use `ArrayHandleSOA` as a default storage without creating
all these useless code paths.
