Should `subsetPixels` support `SpectralImagingArrays`?

It currently breaks without a helpful error message.

It is currently defined as:

```
x[, pixels(x, ..., env = parent.frame(1))]
```

We can support `SpectralImagingArrays` and similar classes with something like:

```
if ( is.null(length(dim(x))) ) {
    x[pixels(x, ..., env = parent.frame(1))]
} else {
    x[,pixels(x, ..., env = parent.frame(1))]
}
```