HDF5Array Object Triggers an Error

The package treats a HDF5Array as not being a kind of array.
```r
> class(patient1)
  "HDF5Array"
  attr(,"package")
  "HDF5Array"
> dim(patient1)
  600 600  40
> writeImage(patient1, file = "~/test.tiff")
  Error in validImage(x) : object must be an array
```